fix(git-tools): pr-create API fallback resolves base from forge default branch (E4) (#1376)

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

@@ -19,6 +19,41 @@ 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 body 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}"
+    # Fetch and parse as separate steps (T51P2WP5AR B2): a piped
+    # `curl | python` reports only python's status, so an HTTP failure that
+    # still emits parseable JSON would masquerade as success. curl's own
+    # exit status is authoritative here.
+    if ! body=$(curl -fsS \
+        -H "User-Agent: curl/8" \
+        -H "Authorization: token ${token}" \
+        "$url" 2>/dev/null); then
+        return 1
+    fi
+    # A valid base is a NONBLANK JSON STRING (T51P2WP5AR B3): null, numbers,
+    # and whitespace-only values are failed resolution, never a POSTed base.
+    branch=$(printf '%s' "$body" | python3 -c '
+import json, sys
+try:
+    value = json.load(sys.stdin).get("default_branch")
+except Exception:
+    sys.exit(1)
+if not isinstance(value, str) or not value.strip():
+    sys.exit(1)
+print(value.strip())
+' 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 +73,28 @@ 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:
@@ -72,7 +121,7 @@ Create a pull request on the current repository (Gitea or GitHub).
 Options:
     -t, --title TITLE       PR title (required, or use --issue)
     -b, --body BODY         PR description/body
-    -B, --base BRANCH       Base branch to merge into (default: main/master)
+    -B, --base BRANCH       Base branch to merge into (default: the forge repository's default branch)
     -H, --head BRANCH       Head branch with changes (default: current branch)
     -l, --labels LABELS     Comma-separated labels
     -m, --milestone NAME    Milestone name