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>

TODOS.md

@@ -338,17 +338,6 @@
 **Effort:** XS
 **Priority:** P2
 
-### Auto-upgrade mode (zero-prompt)
-
-**What:** `GSTACK_AUTO_UPGRADE=1` env var or `~/.gstack/config` option that skips the AskUserQuestion prompt and upgrades automatically.
-
-**Why:** Power users and CI environments want zero-friction upgrades.
-
-**Context:** Current upgrade system (v0.3.4) always prompts. This adds opt-in bypass. ~10 lines in preamble instructions.
-
-**Effort:** S
-**Priority:** P3
-
 ### Eval web dashboard
 
 **What:** `bun run eval:dashboard` serves local HTML with charts: cost trending, detection rate, pass/fail history.
@@ -395,3 +384,7 @@
 ### E2E test cost tracking
 - Track cumulative API spend, warn if over threshold
 **Completed:** v0.3.6
+
+### Auto-upgrade mode + smart update check
+- Config CLI (`bin/gstack-config`), auto-upgrade via `~/.gstack/config.yaml`, 12h cache TTL, exponential snooze backoff (24h→48h→1wk), "never ask again" option, vendored copy sync on upgrade
+**Completed:** v0.3.8