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>

skills/engineering/ask-matt/SKILL.md

@@ -65,6 +65,7 @@ Off the main flow entirely.
 
 - **`/grill-me`** — the same relentless interview as `/grill-with-docs`, but for when you have **no codebase**. Stateless: it saves nothing locally, builds no `CONTEXT.md`. Reach for it to sharpen any plan or design that doesn't live in a repo.
 - **`/prototype`** — a small, throwaway program that answers one design question: does this state model feel right, or what should this UI look like. Throwaway from day one — keep the answer, delete the code. It's the detour in step 2 of the main flow, but reach for it any time a design question is hard to settle on paper.
+- **`/research`** — delegate reading legwork to a **background agent**: it investigates a question against **primary sources**, then leaves a cited Markdown file in the repo. Keep working while it reads. The file it produces is something to take *into* the main flow at `/grill-with-docs` — research feeds the thinking, it doesn't replace it.
 - **`/teach`** — learn a concept over multiple sessions, using the current directory as a stateful workspace.
 - **`/writing-great-skills`** — reference for writing and editing skills well.