framework tools/git: issue-list, milestone-create, lane-brief R1/R4 conversions (P1 complete)

- issue-list + milestone-create: usage() default exit 2, value guards,
  missing-title via usage_error.
- lane-brief: getopts replaced with a while/case parser carrying R2
  long-flag aliases (--repo --milestone --label/--labels --login
  --limit) beside the short forms; login-resolution failure reclassified
  from exit 2 to exit 1 (credential class, not an invocation defect).
- Three suites enrolled (population 88); lane-brief pr-linkage suite
  still ALL PASS; green in both trees; mirrored to the brain tree.

With this, every agent-facing wrapper in tools/git carries the R1/R4
usage-error contract (22/22 non-guard wrappers; guards keep their own
contracts).

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

@@ -36,33 +36,46 @@ Examples:
     $(basename "$0") -m "0.2.0"               # Issues in milestone 0.2.0
     $(basename "$0") --repo ddk/ai-bma         # List issues from anywhere
 EOF
-    exit "${1:-1}"
+    exit "${1:-2}"
 }
 
 # Parse arguments
+# 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
+}
+
 while [[ $# -gt 0 ]]; do
     case $1 in
         -s|--state)
+            [[ $# -ge 2 ]] || usage_error "option $1 requires a value"
             STATE="$2"
             shift 2
             ;;
         -l|--label)
+            [[ $# -ge 2 ]] || usage_error "option $1 requires a value"
             LABEL="$2"
             shift 2
             ;;
         -m|--milestone)
+            [[ $# -ge 2 ]] || usage_error "option $1 requires a value"
             MILESTONE="$2"
             shift 2
             ;;
         -a|--assignee)
+            [[ $# -ge 2 ]] || usage_error "option $1 requires a value"
             ASSIGNEE="$2"
             shift 2
             ;;
         -n|--limit)
+            [[ $# -ge 2 ]] || usage_error "option $1 requires a value"
             LIMIT="$2"
             shift 2
             ;;
         -r|--repo)
+            [[ $# -ge 2 ]] || usage_error "option $1 requires a value"
             REPO_OVERRIDE="$2"
             shift 2
             ;;