feat: smart update check with auto-upgrade, snooze backoff, config CLI (v0.3.9) (#62)

* feat: add bin/gstack-config CLI for reading/writing ~/.gstack/config.yaml

Simple get/set/list interface for persistent gstack configuration.
Used by update-check and upgrade skill for auto_upgrade and update_check settings.

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

* feat: smart update check with 12h cache, snooze backoff, config disable

- Reduce cache TTL from 24h to 12h for faster update detection
- Add exponential snooze backoff: 24h → 48h → 1 week (resets on new version)
- Add update_check: false config option to disable checks entirely
- Clear snooze file on just-upgraded
- 14 new tests covering snooze levels, expiry, corruption, and config paths

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

* feat: upgrade skill with auto-upgrade, 4-option prompt, vendored sync

- Auto-upgrade mode via config or GSTACK_AUTO_UPGRADE=1 env var
- 4-option AskUserQuestion: upgrade once, always, not now, never
- Step 4.5: sync local vendored copy after upgrading primary install
- Snooze write with escalating backoff on "Not now"
- Update preamble text in gen-skill-docs for new upgrade flow
- Regenerate all SKILL.md files

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

* docs: simplify upgrade instructions, move auto-upgrade to completed

README now points to /gstack-upgrade instead of long paste commands.
Auto-upgrade TODO moved to Completed section (v0.3.8).

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

* chore: bump version and changelog (v0.3.9)

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

---------

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

README.md

@@ -598,18 +598,16 @@ Run `cd ~/.claude/skills/gstack && ./setup` (or `cd .claude/skills/gstack && ./s
 Run `cd ~/.claude/skills/gstack && bun install && bun run build`. This compiles the browser binary. Requires Bun v1.0+.
 
 **Project copy is stale?**
-Re-copy from global: `for s in browse plan-ceo-review plan-eng-review review ship retro qa setup-browser-cookies; do rm -f .claude/skills/$s; done && rm -rf .claude/skills/gstack && cp -Rf ~/.claude/skills/gstack .claude/skills/gstack && rm -rf .claude/skills/gstack/.git && cd .claude/skills/gstack && ./setup`
+Run `/gstack-upgrade` — it updates both the global install and any vendored project copy automatically.
 
 **`bun` not installed?**
 Install it: `curl -fsSL https://bun.sh/install | bash`
 
 ## Upgrading
 
-Paste this into Claude Code:
+Run `/gstack-upgrade` in Claude Code. It detects your install type (global or vendored), upgrades, syncs any project copies, and shows what's new.
 
-> Update gstack: run `cd ~/.claude/skills/gstack && git fetch origin && git reset --hard origin/main && ./setup`. If this project also has gstack at .claude/skills/gstack, update it too: run `for s in browse plan-ceo-review plan-eng-review review ship retro qa setup-browser-cookies; do rm -f .claude/skills/$s; done && rm -rf .claude/skills/gstack && cp -Rf ~/.claude/skills/gstack .claude/skills/gstack && rm -rf .claude/skills/gstack/.git && cd .claude/skills/gstack && ./setup`
-
-The `setup` script rebuilds the browser binary and re-symlinks skills. It takes a few seconds.
+Or set `auto_upgrade: true` in `~/.gstack/config.yaml` to upgrade automatically whenever a new version is available.
 
 ## Uninstalling