framework tools/git: P1b — read-only viewers join the R1/R4 contract (pr-diff, pr-view, pr-metadata, pr-list)

usage_error contract (stderr + exit 2), strict option-like value
guards, missing -n checks; pr-list usage() default exit 2. Four suites
enrolled (population 93). Closes the viewer gap from the #1464 census:
carriers now 20 of the 22; remaining non-carriers are the deliberate
design-first class (ci-queue-wait, pr-merge, pr-ci-wait, mosaic-worktree).

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

@@ -13,21 +13,33 @@ OUTPUT_FILE=""
 REPO_OVERRIDE=""
 HOST_OVERRIDE=""
 
+# Usage-error contract (R4): usage errors print to STDERR and exit 2,
+# distinct from provider, credential, and verification failures (exit 1).
+usage_error() {
+    echo "Error: $*" >&2
+    echo "Usage: pr-diff.sh -n <pr_number> [-r owner/repo] [--host host] [-o <output_file>] (see --help)" >&2
+    exit 2
+}
+
 while [[ $# -gt 0 ]]; do
     case $1 in
         -n|--number)
+            [[ $# -ge 2 && "$2" != - && "$2" != --* && ! "$2" =~ ^-[[:alnum:]] ]] || usage_error "option $1 requires a value (option-like values are rejected; bare - is reserved)"
             PR_NUMBER="$2"
             shift 2
             ;;
         -o|--output)
+            [[ $# -ge 2 && "$2" != - && "$2" != --* && ! "$2" =~ ^-[[:alnum:]] ]] || usage_error "option $1 requires a value (option-like values are rejected; bare - is reserved)"
             OUTPUT_FILE="$2"
             shift 2
             ;;
         -r|--repo)
+            [[ $# -ge 2 && "$2" != - && "$2" != --* && ! "$2" =~ ^-[[:alnum:]] ]] || usage_error "option $1 requires a value (option-like values are rejected; bare - is reserved)"
             REPO_OVERRIDE="$2"
             shift 2
             ;;
         --host)
+            [[ $# -ge 2 && "$2" != - && "$2" != --* && ! "$2" =~ ^-[[:alnum:]] ]] || usage_error "option $1 requires a value (option-like values are rejected; bare - is reserved)"
             HOST_OVERRIDE="$2"
             shift 2
             ;;
@@ -40,18 +52,18 @@ while [[ $# -gt 0 ]]; do
             echo "      --host     Gitea host for --repo API calls (or set GITEA_HOST/GITEA_URL)"
             echo "  -o, --output   Output file (optional, prints to stdout if omitted)"
             echo "  -h, --help     Show this help"
+            echo ""
+            echo "Exit codes: 0 success; 2 usage error (stderr); 1 provider/credential failure."
             exit 0
             ;;
         *)
-            echo "Unknown option: $1"
-            exit 1
+            usage_error "unknown option: $1"
             ;;
     esac
 done
 
 if [[ -z "$PR_NUMBER" ]]; then
-    echo "Error: PR number is required (-n)" >&2
-    exit 1
+    usage_error "PR number is required (-n/--number)"
 fi
 
 if [[ -n "$REPO_OVERRIDE" ]]; then