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>

AGENTS.md

@@ -21,6 +21,7 @@ Invoke them by name (e.g., `/office-hours`).
 | `/plan-tune` | Self-tune AskUserQuestion sensitivity per question. |
 | `/autoplan` | One command runs CEO → design → eng → DX review. |
 | `/design-consultation` | Build a complete design system from scratch. |
+| `/spec` | Turn vague intent into a precise, executable spec in five phases. Files a GitHub issue, optionally spawns a Claude Code agent in a fresh worktree, and lets `/ship` close the source issue on merge. |
 
 ### Implementation + review