feat: introduce new skills and refine existing ones

- Add `ask-matt` skill for user-invoked routing.
- Introduce `codebase-design` and `domain-modeling` skills, updating dependencies.
- Remove unused `caveman` and `zoom-out` skills.
- Rename `diagnose` skill to `diagnosing-bugs`.
- Add `resolving-merge-conflicts` skill for handling git conflicts.
- Tighten `review` skill with improved checks.
- Update skill taxonomy to user-invoked and model-invoked.
- Replace `write-a-skill` with `writing-great-skills` for better guidance.

.changeset/codebase-design-and-domain-modeling.md

@@ -0,0 +1,13 @@
+---
+"mattpocock-skills": major
+---
+
+Add the shared design skills and rewire existing skills onto them.
+
+- New **`codebase-design`** skill — the deep-module vocabulary (module, interface, depth, seam, adapter) and the principles for putting a lot of behaviour behind a small interface. The language that previously lived in `improve-codebase-architecture/LANGUAGE.md` now lives here, generalized for reuse across skills.
+- New **`domain-modeling`** skill — actively build and sharpen a project's domain model, stress-testing terms against the glossary and keeping `CONTEXT.md` and ADRs current.
+- `improve-codebase-architecture` now draws its architecture vocabulary from `/codebase-design` and its domain model from `/domain-modeling`.
+- `tdd` now leans on `/codebase-design` for interface-design guidance — its inline `deep-modules.md` / `interface-design.md` notes were removed in favour of the shared skill.
+- `grill-with-docs` now builds the domain model inline via `/domain-modeling`.
+
+**Breaking:** these skills now depend on the new `codebase-design` / `domain-modeling` skills, so you must install them too.