feat: daily update check + /gstack-upgrade skill (v0.3.4) (#42)

* feat: add daily update check script + /gstack-upgrade skill

bin/gstack-update-check: pure bash, checks VERSION against remote once/day,
outputs UPGRADE_AVAILABLE or JUST_UPGRADED. Uses ~/.gstack/ for state.

gstack-upgrade/SKILL.md: new skill with inline upgrade flow for all preambles.
Detects global-git, local-git, vendored installs. Shows What's New from CHANGELOG.

browse/test/gstack-update-check.test.ts: 10 test cases covering all branch paths.

* refactor: remove version check from find-browse, simplify to binary locator

Delete checkVersion(), readCache(), writeCache(), fetchRemoteSHA(),
resolveSkillDir(), CacheEntry interface, REPO_URL/CACHE_PATH/CACHE_TTL
constants, and META output from find-browse.ts.

Version checking is now handled by bin/gstack-update-check (previous commit).

* feat: add update check preamble to all 9 skills

Every skill now runs bin/gstack-update-check on invocation. If an upgrade
is available, reads gstack-upgrade/SKILL.md inline upgrade flow.

Also adds AskUserQuestion to 5 skills that lacked it (gstack root, browse,
qa, retro, setup-browser-cookies) and Bash to plan-eng-review.

Simplifies qa and setup-browser-cookies setup blocks (removes META parsing).

* chore: bump version and changelog (v0.3.4)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: remove unused import + add corrupt cache test

Address pre-landing review findings:
- Remove unused mkdirSync import from gstack-update-check.test.ts
- Add Path I test: corrupt cache file falls through to remote fetch

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>