chore: condense changesets and drop docs-only entries

Removes the five docs-page-only changesets (quickstart drop, grill-me pass,
four-section spine, rewrite-to-standard, writing-for-agents pass) — the
changelog reports on the shipped skills, not on the site pages.

Merges the clusters that describe one change each: ask-matt routing,
the grilling round-by-round rework, prototype, wayfinder, wizard and
writing-for-agents. 27 changesets become 15.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.changeset/writing-for-agents.md

@@ -0,0 +1,9 @@
+---
+"mattpocock-skills": minor
+---
+
+**Breaking:** rename **`writing-great-skills`** → **`writing-for-agents`**, restructure it, and add a new leading word.
+
+The reference now covers any document an agent consumes — skills, `AGENTS.md` / `CLAUDE.md`, docs reached by a pointer — not just skills. `GLOSSARY.md` is merged into `SKILL.md` (one authoritative treatment per term; the `_Avoid_` synonym lists and the standalone Predictability definition are gone); the skill-only mechanics (frontmatter, model- vs user-invoked, router skills, the invocation cut of splitting) are disclosed to a new `SKILL-MECHANICS.md`. The skill is now **model-invoked**: it fires when creating or editing skills or modifying `AGENTS.md`/`CLAUDE.md`. `ask-matt`'s pointer updated. Reinstall under the new name; the old name is gone (no alias).
+
+The pruning section gains **cache**. Single source of truth now reaches past the document into the environment — `package.json` scripts, config files, directory layout, `--help` output are themselves authoritative, so a doc that restates them is a cache of a lookup, earning its load only when the lookup is expensive. The positive target: cache what the agent cannot find by looking (unwritten conventions, the reason behind a choice, gotchas no config confesses), and leave one-file, one-command lookups to the environment, where they cannot go stale.