chore(agents): install mattpocock skills + per-repo agent config

- 38 skills from mattpocock/skills into .agents/skills/ (symlinked into
  .claude/skills/ for Claude Code)
- skills-lock.json for reproducible installs
- docs/agents/: issue-tracker (Gitea via tea CLI), triage-labels
  (default five-role vocabulary), domain docs (single-context)
- CLAUDE.md: add '## Agent skills' section pointing at docs/agents/
- .gitignore: un-ignore docs/agents/ under the local /docs/* rule

.agents/skills/pr/CREDITS.md

@@ -0,0 +1,3 @@
+# Credits
+
+"The shape of the change" section is [Dex Horthy](https://github.com/dexhorthy)'s [`show-me`](https://github.com/humanlayer/humanlayer) skill, reproduced almost word for word, aimed at a diff instead of a live conversation. `pr` does not depend on `show-me` as a skill (it isn't part of this repo, and a hard dependency would break standalone installs), so the content is copied in rather than pointed at; this file is the attribution a dependency would otherwise have carried.