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/setup-matt-pocock-skills/issue-tracker-local.md

@@ -0,0 +1,30 @@
+# Issue tracker: Local Markdown
+
+Issues and specs for this repo live as markdown files in `.scratch/`.
+
+## Conventions
+
+- One feature per directory: `.scratch/<feature-slug>/`
+- The spec is `.scratch/<feature-slug>/spec.md`
+- Implementation issues are one file per ticket at `.scratch/<feature-slug>/issues/<NN>-<slug>.md`, numbered from `01`, never a single combined tickets file
+- Triage state is recorded as a `Status:` line near the top of each issue file (see `triage-labels.md` for the role strings)
+- Comments and conversation history append to the bottom of the file under a `## Comments` heading
+
+## When a skill says "publish to the issue tracker"
+
+Create a new file under `.scratch/<feature-slug>/` (creating the directory if needed).
+
+## When a skill says "fetch the relevant ticket"
+
+Read the file at the referenced path. The user will normally pass the path or the issue number directly.
+
+## Wayfinding operations
+
+Used by `/wayfinder`. The **map** is a file with one **child** file per ticket.
+
+- **Map**: `.scratch/<effort>/map.md` (the Notes / Decisions-so-far / Fog body).
+- **Child ticket**: `.scratch/<effort>/issues/NN-<slug>.md`, numbered from `01`, with the question in the body. A `Type:` line records the ticket type (`research`/`prototype`/`grilling`/`task`); a `Status:` line records `claimed`/`resolved`.
+- **Blocking**: a `Blocked by: NN, NN` line near the top. A ticket is unblocked when every file it lists is `resolved`.
+- **Frontier**: scan `.scratch/<effort>/issues/` for files that are open, unblocked, and unclaimed; first by number wins.
+- **Claim**: set `Status: claimed` and save before any work.
+- **Resolve**: append the answer under an `## Answer` heading, set `Status: resolved`, then append a context pointer (gist + link) to the map's Decisions-so-far in `map.md`.