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/implement/SKILL.md

@@ -0,0 +1,15 @@
+---
+name: implement
+description: "Implement a piece of work based on a spec or set of tickets."
+disable-model-invocation: true
+---
+
+Implement the work described by the user in the spec or tickets.
+
+Use /tdd where possible, at pre-agreed seams.
+
+Run typechecking regularly, single test files regularly, and the full test suite once at the end.
+
+Once done, use /code-review to review the work.
+
+Commit your work to the current branch.