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/research/SKILL.md

@@ -0,0 +1,12 @@
+---
+name: research
+description: Investigate a question against high-trust primary sources and capture the findings as a Markdown file in the repo. Use when the user wants a topic researched, docs or API facts gathered, or reading legwork delegated to a background agent.
+---
+
+Spin up a **background agent** to do the research, so you keep working while it reads. Start it with a fresh context window.
+
+Its job:
+
+1. Investigate the question against **primary sources** (official docs, source code, specs, first-party APIs), not a secondary write-up of them. Follow every claim back to the source that owns it.
+2. Write the findings to a single Markdown file, citing each claim's source.
+3. Save it where the repo already keeps such notes; match the existing convention, and if there is none, put it somewhere sensible and say where.