feat: add the research skill

A small, model-invoked skill that spins up a background agent to
investigate a question against high-trust primary sources, then leaves a
single cited Markdown file wherever the repo keeps such notes.

Wires it into the plugin set, both READMEs (Model-invoked), a docs page,
and the ask-matt router, per the repo's promoted-skill conventions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -161,6 +161,7 @@ Skills I use daily for code work.
 
 - **[prototype](./skills/engineering/prototype/SKILL.md)** — Build a throwaway prototype to answer a design question — a runnable terminal app for state/logic questions, or several radically different UI variations toggleable from one route.
 - **[diagnosing-bugs](./skills/engineering/diagnosing-bugs/SKILL.md)** — Disciplined diagnosis loop for hard bugs and performance regressions: reproduce → minimise → hypothesise → instrument → fix → regression-test.
+- **[research](./skills/engineering/research/SKILL.md)** — Investigate a question against high-trust primary sources and capture the findings as a cited Markdown file in the repo, run as a background agent.
 - **[tdd](./skills/engineering/tdd/SKILL.md)** — Test-driven development with a red-green-refactor loop. Builds features or fixes bugs one vertical slice at a time.
 - **[domain-modeling](./skills/engineering/domain-modeling/SKILL.md)** — Actively build and sharpen a project's domain model — challenge terms against the glossary, stress-test with edge-case scenarios, and update `CONTEXT.md` and ADRs inline.
 - **[codebase-design](./skills/engineering/codebase-design/SKILL.md)** — Shared discipline and vocabulary for designing deep modules: a lot of behaviour behind a small interface, placed at a clean seam, testable through that interface.