gsd-check-update.js scanned ALL .js files in the hooks directory and
flagged any without a gsd-hook-version header as stale. This incorrectly
flagged user-created hooks (e.g. guard-edits-outside-project.js),
producing a persistent 'stale hooks' warning that /gsd:update couldn't
resolve.
Fix: filter hookFiles to f.startsWith('gsd-') && f.endsWith('.js')
since all GSD hooks follow the gsd-* naming convention.
Includes regression test validating the filter excludes user hooks.
Closes#1200
9ca03ec35e