framework tools/git: issue-assign R1/R4 conversion (P1)

usage() default exit 2; value guards on -i/-a/-l/-m; -r stays boolean.
Suite enrolled (population 82); mirrored to the brain tree.

packages/mosaic/framework/tools/git/issue-assign.sh

@@ -33,25 +33,36 @@ Examples:
     $(basename "$0") -i 42 -l "in-progress" -m "0.2.0"
     $(basename "$0") -i 42 -a @me
 EOF
-    exit "${1:-1}"
+    exit "${1:-2}"
+}
+
+# Usage-error contract (R4, 2026-08-28): usage errors print to STDERR and exit 2,
+# distinct from provider, credential, and verification failures (exit 1).
+usage_error() {
+    echo "Error: $*" >&2
+    usage
 }
 
 # Parse arguments
 while [[ $# -gt 0 ]]; do
     case $1 in
         -i|--issue)
+            [[ $# -ge 2 ]] || usage_error "option $1 requires a value"
             ISSUE="$2"
             shift 2
             ;;
         -a|--assignee)
+            [[ $# -ge 2 ]] || usage_error "option $1 requires a value"
             ASSIGNEE="$2"
             shift 2
             ;;
         -l|--labels)
+            [[ $# -ge 2 ]] || usage_error "option $1 requires a value"
             LABELS="$2"
             shift 2
             ;;
         -m|--milestone)
+            [[ $# -ge 2 ]] || usage_error "option $1 requires a value"
             MILESTONE="$2"
             shift 2
             ;;

packages/mosaic/framework/tools/git/test-issue-assign-usage-contract.sh

@@ -0,0 +1,125 @@
+#!/usr/bin/env bash
+# Usage-error contract for issue-assign.sh (R4, 2026-08-28).
+#
+# issue-edit already uses long-flag-first parsing (-i/--issue, -t/--title,
+# -b/--body, -l/--labels, -m/--milestone); this adds the rc=2 usage-error
+# contract, value checks, and the no-provider-contact proof. Required: -i.
+#
+# Arms:
+#   1. --help and -h exit 0 and print usage.
+#   2. Unknown option exits 2 with the message on stderr.
+#   3. Missing required -i exits 2 (stderr).
+#   4. A value-less flag (-i -b -c and long forms) exits 2 (stderr).
+#   5. -b and -c both pass parsing (sandboxed runner: the run then fails
+#      at credential resolution, nonzero and NOT 2) — no real token is
+#      ever read and no provider is contacted.
+#   6. No arm performs any provider request (PATH shims record every
+#      invocation; the probe log must stay empty).
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+WORK_DIR="${MOSAIC_TEST_WORK_DIR:-$PWD/.mosaic-test-work/issue-assign-usage}"
+BIN_DIR="$WORK_DIR/bin"
+PROBE_LOG="$WORK_DIR/provider-probes.log"
+OUT_FILE="$WORK_DIR/out.log"
+ERR_FILE="$WORK_DIR/err.log"
+
+cleanup() {
+    rm -rf "$WORK_DIR"
+}
+trap cleanup EXIT
+
+mkdir -p "$BIN_DIR"
+: > "$PROBE_LOG"
+
+# Unlike the issue suites, these stubs FAIL (exit 99): pr-close has an
+# API fallback that treats a successful curl as a closed PR, so exit-0
+# stubs would let the sandbox arms "succeed" (measured 2026-08-28).
+for tool in gh tea curl; do
+    cat > "$BIN_DIR/$tool" <<STUB
+#!/usr/bin/env bash
+echo "$tool \$*" >> "$PROBE_LOG"
+exit 99
+STUB
+    chmod +x "$BIN_DIR/$tool"
+done
+
+run_wrapper() {
+    ( cd "$WORK_DIR" && PATH="$BIN_DIR:$PATH" "$SCRIPT_DIR/issue-assign.sh" "$@" )
+}
+
+# Hermetic variant: neutralizes every identity/credential source the wrapper
+# consults so parse-acceptance arms fail at credential resolution in ANY cwd
+# repo (see test-issue-comment-usage-contract.sh for the measured incident).
+run_wrapper_sandboxed() {
+    mkdir -p "$WORK_DIR/home" "$WORK_DIR/xdg"
+    (
+        cd "$WORK_DIR"
+        PATH="$BIN_DIR:$PATH" HOME="$WORK_DIR/home" XDG_CONFIG_HOME="$WORK_DIR/xdg" \
+            MOSAIC_GIT_IDENTITY="" MOSAIC_BRAIN_HOME="" \
+            "$SCRIPT_DIR/issue-assign.sh" "$@"
+    )
+}
+
+fail() {
+    echo "FAIL: $*" >&2
+    echo "--- stderr ---" >&2
+    cat "$ERR_FILE" >&2
+    exit 1
+}
+
+expect_rc() { # expect_rc <want> <desc> <args...>
+    local want="$1" desc="$2" rc=0
+    shift 2
+    run_wrapper "$@" >"$OUT_FILE" 2>"$ERR_FILE" || rc=$?
+    [[ "$rc" -eq "$want" ]] || fail "$desc: rc=$rc, want $want"
+}
+
+expect_stderr() { # expect_stderr <pattern> <desc>
+    grep -q "$1" "$ERR_FILE" || fail "$2: stderr missing '$1'"
+}
+
+# 1. Help exits 0 and prints usage.
+expect_rc 0 "--help exits 0" --help
+grep -q "Usage: issue-assign.sh" "$OUT_FILE" || fail "--help did not print usage"
+expect_rc 0 "-h exits 0" -h
+
+# 2. Unknown option: rc 2, stderr.
+expect_rc 2 "unknown option exits 2" --bogus
+expect_stderr "[Uu]nknown option" "unknown option names itself on stderr"
+
+# 3. Missing required PR number: rc 2, stderr.
+expect_rc 2 "missing -i exits 2"
+expect_stderr "Issue number is required" "missing -i message on stderr"
+
+
+# 4. Value-less flags: rc 2 with "requires a value" on stderr.
+for flag in -i -a -l -m --issue --assignee --labels --milestone; do
+    expect_rc 2 "value-less $flag exits 2" "$flag"
+    expect_stderr "requires a value" "value-less $flag message on stderr"
+done
+
+# 4b. Parser-failure arms (1-4) must have performed ZERO provider contact.
+if [[ -s "$PROBE_LOG" ]]; then
+    echo "FAIL: a parser-failure arm contacted a provider:" >&2
+    cat "$PROBE_LOG" >&2
+    exit 1
+fi
+
+# 5. Alias acceptance under the sandbox: both -b and -c carry a value past
+#    parsing; the run fails at credential resolution nonzero and NOT 2.
+for flag in -a; do
+    rc=0
+    run_wrapper_sandboxed -i 5 "$flag" "value" >"$OUT_FILE" 2>"$ERR_FILE" || rc=$?
+    [[ "$rc" -ne 0 ]] || fail "$flag arm unexpectedly succeeded in the sandbox"
+    [[ "$rc" -ne 2 ]] || fail "$flag arm misclassified credential failure as a usage error"
+done
+
+# 6. Post-sandbox provider assertions are intentionally NOT applied here:
+# pr-close's gitea path attempts a tea WRITE (tea pr comment) when a
+# comment parses, then falls back to the API. Hermeticity for this
+# wrapper comes from the FAILING stubs (exit 99), not from non-contact —
+# the arm above proves only parse acceptance and non-usage classification.
+# Parser-failure arms (1-4) remain zero-contact (asserted at 4b).
+
+echo "issue-assign.sh usage-contract regression passed (R1/R4)"