fix(scripts): make the mirror DRIFT fix line safe to copy-paste

Why: bd2bf66 restored the `apm pack` guard-rail by appending it to the `Fix:`
command after `--`, which made the printed line stop being runnable. Pasting it
ran the script with ~24 stray argv entries: `${1:-}` became `--`, so CHECK
stayed 0, no shift occurred, and `[[ $# -eq 0 ]] || usage` printed usage and
exited 1. The user got a usage error from the tool meant to fix their problem,
and the mirror stayed stale.

The unquoted backticks around `apm pack` were a second hazard in the same line:
the paste command-substituted a real `apm pack` run before this script was ever
reached, so the first error a user saw came from apm, not from here.

Implementation notes:
- The runnable command now stands alone on its own line, and the rationale
  follows as a separate `Note:` echo.
- Backticks downgraded to single quotes; a line printed next to a
  copy-pasteable command must not contain shell metacharacters.
- The guard-rail text is otherwise preserved verbatim. It exists because apm
  ships no output profile targeting this path, so `apm pack` does not refresh
  it, and expecting it to is the drift this hook prevents.

Impact: reproduced the break on a scratch copy, then verified the fix by pasting
the printed command verbatim — exit 0, mirror synced, re-check clean.
tests/test-sync-marketplace-mirror.sh asserts only exit codes and file contents,
so nothing pins this message and it could regress silently; tracked separately.

Refs: #105

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TmFqzpuExLJv3m114XVE9w

scripts/sync-marketplace-mirror.sh

@@ -60,7 +60,19 @@ fi
 if [[ "$CHECK" -eq 1 ]]; then
   if [[ ! -f "$DST" ]] || ! diff -q "$SRC" "$DST" >/dev/null 2>&1; then
     echo "DRIFT $DST: out of sync with .claude-plugin/marketplace.json" >&2
-    echo "Fix: bash scripts/sync-marketplace-mirror.sh -- apm ships no output profile targeting this path, so \`apm pack\` does not refresh it. Expecting it to is exactly the drift this script and its pre-push hook exist to prevent." >&2
+    # The runnable command gets a line to ITSELF, and the rationale gets its own
+    # echo. It was one line -- `Fix: bash scripts/sync-marketplace-mirror.sh --
+    # apm ships no output profile...` -- which put the prose after `--`, the
+    # POSIX end-of-options marker, so copy-pasting the Fix line ran this script
+    # with ~24 stray argv entries: `${1:-}` was `--` (so CHECK stayed 0 and no
+    # shift happened), `[[ $# -eq 0 ]]` failed, and the tool meant to fix the
+    # drift answered with its own usage error and exit 1. The backticks around
+    # `apm pack` made it worse: the paste also command-substituted a real
+    # `apm pack` run before the script was even reached. Hence plain quotes
+    # below too. Keep the command alone on its line.
+    echo "Fix: run, from the repository root:" >&2
+    echo "  bash scripts/sync-marketplace-mirror.sh" >&2
+    echo "Note: apm ships no output profile targeting this path, so 'apm pack' does not refresh it. Expecting it to is exactly the drift this script and its pre-push hook exist to prevent." >&2
     exit 1
   fi
   exit 0