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>

CLAUDE.md

@@ -111,6 +111,7 @@ gstack/
 ├── land-and-deploy/ # /land-and-deploy skill (merge → deploy → canary verify)
 ├── office-hours/    # /office-hours skill (YC Office Hours — startup diagnostic + builder brainstorm)
 ├── investigate/     # /investigate skill (systematic root-cause debugging)
+├── spec/            # /spec skill (five-phase spec → GitHub issue, optional agent spawn, /ship auto-closes)
 ├── retro/           # Retrospective skill (includes /retro global cross-project mode)
 ├── bin/             # CLI utilities (gstack-repo-mode, gstack-slug, gstack-config, etc.)
 ├── document-release/ # /document-release skill (post-ship doc updates + Diataxis coverage map)