fix(git-tools): pr-create API fallback resolves base from forge default branch (E4)

The API-fallback payload hardcoded "base": "main", mistargeting every
fallback PR on repos whose trunk is not main. With zero fleet tea logins
(measured under T53) every pr-create takes the fallback, so stack repos
(default branch next) got main-targeted PRs.

gitea_pr_create_api now resolves the base when -B is omitted:
gitea_default_branch() queries the provider repo API
(GET /api/v1/repos/<owner>/<repo> -> default_branch) using the same
host/repo/token resolution as the POST itself. Explicit -B still wins and
skips the lookup entirely. A failed or empty resolution fails loud
(-B remedy named) instead of guessing a base; the "main" literal is gone.

Hermetic suite test-pr-create-fallback-default-base.sh (PATH-stubbed curl,
sandbox HOME, git.example.test fixture): no -B -> payload base equals the
stubbed forge default (next) with the repo GET performed; explicit -B wins
with zero repo GETs; lookup failure -> loud refusal, no POST; the literal
main never appears as a fallback. Fixture stubs in
test-gitea-login-resolution.sh and test-issue-create-interactive-auth.sh
gain repo-root answers (assertions unchanged) because the fallback now
makes a call those stubs predated.

Sweep: 32/33 git-tools suites green; test-issue-close-fail-closed.sh fails
identically at baseline origin/next (pre-existing, untouched).

packages/mosaic/framework/tools/git/pr-create.sh

@@ -19,6 +19,24 @@ ISSUE=""
 
 # get_remote_host, get_gitea_token, get_repo_info, and get_gitea_repo_args are provided by detect-platform.sh
 
+gitea_default_branch() {
+    # Forge default branch for the current repo (T51-P2 WP5a / spec E4): the
+    # API fallback must not guess a base. Empty output or any lookup failure
+    # returns nonzero so the caller fails loud instead of mistargeting a PR.
+    local host repo token url branch
+    host=$(get_remote_host) || return 1
+    repo=$(get_repo_info) || return 1
+    token=$(get_gitea_token "$host") || return 1
+    url="https://${host}/api/v1/repos/${repo}"
+    branch=$(curl -fsS \
+        -H "User-Agent: curl/8" \
+        -H "Authorization: token ${token}" \
+        "$url" \
+        | python3 -c 'import json,sys; print(json.load(sys.stdin).get("default_branch",""))' 2>/dev/null) || return 1
+    [[ -n "$branch" ]] || return 1
+    printf '%s' "$branch"
+}
+
 gitea_pr_create_api() {
     local host repo token url payload
     host=$(get_remote_host) || {
@@ -38,14 +56,28 @@ gitea_pr_create_api() {
         echo "Warning: API fallback applies title/body/head/base only; labels/milestone/draft require authenticated tea setup." >&2
     fi
 
-    payload=$(TITLE="$TITLE" BODY="$BODY" HEAD_BRANCH="$HEAD_BRANCH" BASE_BRANCH="$BASE_BRANCH" python3 - <<'PY'
+    # Base resolution (spec E4): an explicit -B always wins; with none, the
+    # forge default branch is resolved from the provider API -- never the
+    # historical "main" literal, which mistargeted every fallback PR on
+    # repos whose trunk is not main (e.g. mosaicstack/stack -> next).
+    local api_base=""
+    if [[ -n "$BASE_BRANCH" ]]; then
+        api_base="$BASE_BRANCH"
+    else
+        api_base=$(gitea_default_branch) || {
+            echo "Error: could not resolve the forge default branch for the API-fallback base; pass -B <branch> explicitly" >&2
+            return 1
+        }
+    fi
+
+    payload=$(TITLE="$TITLE" BODY="$BODY" HEAD_BRANCH="$HEAD_BRANCH" API_BASE="$api_base" python3 - <<'PY'
 import json
 import os
 
 payload = {
     "title": os.environ["TITLE"],
     "head": os.environ["HEAD_BRANCH"],
-    "base": os.environ["BASE_BRANCH"] or "main",
+    "base": os.environ["API_BASE"],
 }
 body = os.environ.get("BODY", "")
 if body:

packages/mosaic/framework/tools/git/test-gitea-login-resolution.sh

@@ -100,6 +100,15 @@ case "$url" in
     */commits/*/status)
         printf '{"state":"success","statuses":[{"context":"ci/mock","status":"success"}]}'
         ;;
+    # Repo roots: the pr-create API fallback resolves its base from the forge
+    # default_branch (T51-P2 WP5a). Exact-suffix matches so the /pulls POST
+    # endpoint (no trailing path) still falls through to the catch-all.
+    */api/v1/repos/USC/uconnect)
+        printf '{"default_branch":"main"}'
+        ;;
+    */api/v1/repos/mosaicstack/stack)
+        printf '{"default_branch":"next"}'
+        ;;
     *)
         printf '{}'
         ;;

packages/mosaic/framework/tools/git/test-issue-create-interactive-auth.sh

@@ -67,7 +67,18 @@ cat > "$BIN_DIR/curl" <<'SH'
 #!/usr/bin/env bash
 set -euo pipefail
 printf 'curl %s\n' "$*" >> "$MOSAIC_TEST_LOG"
-printf '%s\n' '{"number":703}'
+# Repo roots: the pr-create API fallback resolves its base from the forge
+# default_branch (T51-P2 WP5a). Exact-suffix so every other endpoint keeps
+# the historical answer below.
+url="${*: -1}"
+case "$url" in
+    */api/v1/repos/mosaicstack/stack)
+        printf '%s\n' '{"default_branch":"next"}'
+        ;;
+    *)
+        printf '%s\n' '{"number":703}'
+        ;;
+esac
 SH
 chmod +x "$BIN_DIR/tea" "$BIN_DIR/curl"
 

packages/mosaic/framework/tools/git/test-pr-create-fallback-default-base.sh

@@ -0,0 +1,140 @@
+#!/usr/bin/env bash
+# test-pr-create-fallback-default-base.sh — hermetic test for the API-fallback
+# base resolution in pr-create.sh (T51-P2 WP5a / spec E4).
+#
+# The API-fallback payload historically hardcoded "base": "main", mistargeting
+# every fallback PR on repos whose trunk is not main (e.g. mosaicstack/stack,
+# default branch "next"). The fix: an explicit -B always wins; with none, the
+# base is resolved from the provider API default_branch, and a failed
+# resolution fails loud instead of guessing.
+#
+# Hermetic by construction: every curl invocation is a PATH-first stub; the
+# fixture repo's remote is git.example.test (never dialed); HOME is a sandbox
+# with no tea config (so the wrapper takes the API fallback path); GITEA_TOKEN
+# comes from the environment. No real forge is contacted.
+
+# shellcheck disable=SC2317
+set -uo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+WORK_DIR="${MOSAIC_TEST_WORK_DIR:-$PWD/.mosaic-test-work/pr-create-fallback-base}"
+
+PASS=0 FAIL=0 FAILED_CASES=""
+
+ok()  { PASS=$((PASS + 1)); }
+bad() { FAIL=$((FAIL + 1)); FAILED_CASES="$FAILED_CASES $1"; printf 'FAIL: %s\n' "$1" >&2; }
+
+assert_rc()    { local d="$1" e="$2" a="$3"; [ "$e" = "$a" ] && ok || bad "$d (expected rc=$e got rc=$a)"; }
+assert_eq()    { local d="$1" e="$2" a="$3"; [ "$e" = "$a" ] && ok || bad "$d (expected [$e] got [$a])"; }
+assert_contains() { local d="$1" h="$2" n="$3"; case "$h" in *"$n"*) ok ;; *) bad "$d (missing [$n])" ;; esac; }
+
+json_field() { # $1 payload file, $2 field
+    python3 -c 'import json,sys; print(json.load(open(sys.argv[1])).get(sys.argv[2], ""))' "$1" "$2"
+}
+
+rm -rf "$WORK_DIR"
+
+# ---- fixture -----------------------------------------------------------------
+ROOT="$WORK_DIR/fixture"
+TOOLS="$ROOT/tools/git"
+mkdir -p "$TOOLS" "$ROOT/repo" "$ROOT/home" "$ROOT/stub"
+cp "$SCRIPT_DIR/pr-create.sh" "$TOOLS/pr-create.sh"
+cp "$SCRIPT_DIR/detect-platform.sh" "$TOOLS/detect-platform.sh"
+
+git -C "$ROOT/repo" init -q -b fix/e4
+git -C "$ROOT/repo" -c user.name=fixture -c user.email=fixture@test commit -q --allow-empty -m base
+git -C "$ROOT/repo" remote add origin https://git.example.test/acme/widgets.git
+
+# curl stub: GET repo -> default_branch JSON (or failure mode); POST pulls ->
+# capture payload, answer with a minimal PR JSON. Every call is logged.
+cat > "$ROOT/stub/curl" <<STUB
+#!/usr/bin/env bash
+set -u
+mode="\${CURL_STUB_GET_MODE:-ok}"
+printf '%s\n' "\$*" >> "$ROOT/curl-calls.log"
+url="\${!#}"
+if [[ "\$url" == */api/v1/repos/acme/widgets ]]; then
+    # repo GET (default-branch resolution)
+    if [[ "\$mode" != "ok" ]]; then
+        echo "curl stub: simulated repo lookup failure" >&2
+        exit 1
+    fi
+    printf '%s\n' '{"id":1,"default_branch":"next","full_name":"acme/widgets"}'
+    exit 0
+fi
+if [[ "\$url" == */api/v1/repos/acme/widgets/pulls ]]; then
+    # PR POST: capture the payload, emit a PR-shaped answer
+    while [[ \$# -gt 0 ]]; do
+        case "\$1" in
+            -d) printf '%s' "\$2" > "$ROOT/payload.json"; shift 2 ;;
+            *) shift ;;
+        esac
+    done
+    printf '%s\n' '{"number":42,"html_url":"https://git.example.test/acme/widgets/pulls/42"}'
+    exit 0
+fi
+echo "curl stub: unexpected URL \$url" >&2
+exit 1
+STUB
+chmod +x "$ROOT/stub/curl"
+
+run_pr_create() { # args... -> sets RC/OUT/ERR
+    RC=0
+    OUT=$(cd "$ROOT/repo" && env -i \
+        PATH="$ROOT/stub:/usr/bin:/bin" \
+        HOME="$ROOT/home" \
+        GITEA_TOKEN=stub-token \
+        bash "$TOOLS/pr-create.sh" "$@" 2>"$ROOT/err.txt")
+    RC=$?
+    ERR="$(cat "$ROOT/err.txt")"
+}
+
+calls_matching() { grep -c -- "$1" "$ROOT/curl-calls.log" 2>/dev/null || true; }
+
+echo "== (1) no -B: fallback base resolves to the forge default branch, not main =="
+: > "$ROOT/curl-calls.log"; rm -f "$ROOT/payload.json"
+run_pr_create -t "fix thing"
+assert_rc "rc" 0 "$RC"
+assert_contains "API fallback path taken (tea login unresolvable in fixture)" "$ERR" "trying Gitea API fallback"
+assert_eq "repo GET performed" 1 "$(calls_matching '/api/v1/repos/acme/widgets$')"
+assert_eq "POST performed" 1 "$(calls_matching '/pulls$')"
+assert_eq "payload base is forge default (next)" "next" "$(json_field "$ROOT/payload.json" base)"
+assert_eq "payload head" "fix/e4" "$(json_field "$ROOT/payload.json" head)"
+assert_eq "payload title" "fix thing" "$(json_field "$ROOT/payload.json" title)"
+
+echo "== (2) explicit -B wins; the default branch is not consulted =="
+: > "$ROOT/curl-calls.log"; rm -f "$ROOT/payload.json"
+run_pr_create -t "fix thing" -B release/1.x
+assert_rc "rc" 0 "$RC"
+assert_eq "repo GET not consulted for explicit base" 0 "$(calls_matching '/api/v1/repos/acme/widgets$')"
+assert_eq "payload base is the explicit -B" "release/1.x" "$(json_field "$ROOT/payload.json" base)"
+
+echo "== (3) default-branch lookup failure: loud refusal, no POST =="
+: > "$ROOT/curl-calls.log"; rm -f "$ROOT/payload.json"
+RC=0
+OUT=$(cd "$ROOT/repo" && env -i \
+    PATH="$ROOT/stub:/usr/bin:/bin" \
+    HOME="$ROOT/home" \
+    GITEA_TOKEN=stub-token \
+    CURL_STUB_GET_MODE=fail \
+    bash "$TOOLS/pr-create.sh" -t "fix thing" 2>"$ROOT/err.txt")
+RC=$?
+ERR="$(cat "$ROOT/err.txt")"
+assert_rc "nonzero rc on unresolvable base" 1 "$RC"
+assert_contains "loud error names -B" "$ERR" "could not resolve the forge default branch"
+assert_contains "error names the remedy" "$ERR" "pass -B <branch> explicitly"
+assert_eq "no POST issued" 0 "$(calls_matching '/pulls$')"
+
+echo "== (4) payload never contains the literal fallback main =="
+: > "$ROOT/curl-calls.log"; rm -f "$ROOT/payload.json"
+run_pr_create -t "fix thing"
+assert_rc "rc" 0 "$RC"
+assert_eq "base field is next, never main" "next" "$(json_field "$ROOT/payload.json" base)"
+
+echo
+echo "pass=$PASS fail=$FAIL"
+if [ "$FAIL" -gt 0 ]; then
+    echo "FAILED CASES:$FAILED_CASES"
+    exit 1
+fi
+echo "ALL GREEN"