feat(ai): add the mattpocock engineering skills for AI agents

Vendor the engineering skills of https://github.com/mattpocock/skills into
ai/skills/engineering, with their MIT license, and list them in AGENTS.md.
The agent-specific metadata (agents/openai.yaml) is kept for other agents.

Additional changes
- Add scripts/update_mattpocock_skills.py to compare the copy with upstream by
  git blob hashes from a blobless clone and to update it on request.
- Add tests/test_ai.py; it warns when the copy differs from upstream and caches
  the upstream hashes in the pytest cache for a day.
- Document the engineering skills in docs/develop/ai.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ai/skills/engineering/implement/SKILL.md

@@ -0,0 +1,17 @@
+---
+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.
+
+If the user passes a ticket reference, fetch it from the issue tracker and state its title before starting. If the reference is ambiguous, ask.
+
+Call the Skill tool with "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, call the Skill tool with "code-review" to review the work.
+
+Commit your work to the current branch.