feat!: rename writing-great-skills to writing-for-agents and restructure

The reference now covers any document an agent consumes — skills,
AGENTS.md/CLAUDE.md, docs reached by a pointer. GLOSSARY.md merged into
SKILL.md as a dedup (Avoid-lists and the standalone Predictability
definition pruned); skill-only mechanics disclosed to SKILL-MECHANICS.md;
the skill is now model-invoked. Clean rename, no alias.

Spec: mattpocock/personal-wiki#187 · ships via mattpocock/personal-wiki#193

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

skills/engineering/ask-matt/SKILL.md

@@ -71,7 +71,7 @@ Off the main flow entirely.
 - **`/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.
+- **`/writing-for-agents`** — reference for writing documents agents consume: skills, AGENTS.md, pointed-at docs.
 
 ## Precondition