docs: register /spec skill in README, AGENTS, CLAUDE.md project tree

Adds /spec to the three discoverability surfaces it was missing:
- README.md sprint skills table (between /autoplan and /learn)
- AGENTS.md plan-mode reviews table
- CLAUDE.md project structure tree (between /investigate and /retro)

/spec shipped in v1.47.0.0 with CHANGELOG coverage but the entry-point
docs hadn't been updated; a user landing on README or AGENTS would not
discover the skill exists without reading CHANGELOG.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -204,6 +204,7 @@ Each skill feeds into the next. `/office-hours` writes a design doc that `/plan-
 | `/browse` | **QA Engineer** | Give the agent eyes. Real Chromium browser, real clicks, real screenshots. ~100ms per command. `/open-gstack-browser` launches GStack Browser with sidebar, anti-bot stealth, and auto model routing. |
 | `/setup-browser-cookies` | **Session Manager** | Import cookies from your real browser (Chrome, Arc, Brave, Edge) into the headless session. Test authenticated pages. |
 | `/autoplan` | **Review Pipeline** | One command, fully reviewed plan. Runs CEO → design → eng review automatically with encoded decision principles. Surfaces only taste decisions for your approval. |
+| `/spec` | **Spec Author** | Turn vague intent into a precise, executable spec in five phases (why, scope, technical with mandatory code-reading, draft, file). Codex quality gate before file (blocks below 7/10), fail-closed secret redaction, dedupe against existing issues, archive to `$GSTACK_STATE_ROOT/projects/$SLUG/specs/` for team-corpus recall. `--execute` spawns `claude -p` in a fresh worktree; `/ship` auto-closes the source issue on merge. Plan-mode aware. |
 | `/learn` | **Memory** | Manage what gstack learned across sessions. Review, search, prune, and export project-specific patterns, pitfalls, and preferences. Learnings compound across sessions so gstack gets smarter on your codebase over time. |
 
 ### Which review should I use?