framework tools/git: issue-create R1/R4 conversion + template bug fix (P1)

- issue-create: usage() default exit is now 2 (usage-error contract);
  unknown option, missing title, and value-less -t/-b/-l/-m land there
  with stderr messages; exit-code line added to help. Interactive
  -i/--interactive unchanged (boolean, no value arm). Existing
  body-safety and interactive-auth suites still green.
- Template bug fixed across ALL seven usage-contract suites:
  expect_stderr referenced an unbound $desc (should be $2); latent
  until issue-create's capitalized 'Unknown option' missed the
  case-sensitive grep and fired it. All seven re-verified green.
- Suite enrolled (population 79); mirrored to the brain tree.

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

@@ -74,26 +74,39 @@ Examples:
     $(basename "$0") -t "Fix login bug" -l "bug,priority-high"
     $(basename "$0") -t "Add dark mode" -b "Implement theme switching" -m "0.2.0"
     $(basename "$0") -i
+
+Exit codes: 0 success; 2 usage error (stderr); 1 provider/credential failure.
 EOF
-    exit "${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
         -t|--title)
+            [[ $# -ge 2 ]] || usage_error "option $1 requires a value"
             TITLE="$2"
             shift 2
             ;;
         -b|--body)
+            [[ $# -ge 2 ]] || usage_error "option $1 requires a value"
             BODY="$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
             ;;