docs: tell one true install story (official marketplace)

mattpocock-skills is now listed in Claude Code's official marketplace,
so the own-marketplace route is superseded. The queued changeset still
documented it, and changeset bodies ship verbatim into CHANGELOG.md —
the release would have published a superseded install route as its
headline.

- Rewrite .changeset/ship-as-claude-plugin.md around
  `claude plugins install mattpocock-skills`.
- Add .agents/install-block.md as the single source for install
  wording, so the docs pass has one true thing to copy.
- Point .agents/writing-docs.md and CLAUDE.md at it.
- Note the official listing in ADR 0002, and record that it reads
  plugin.json directly and does not depend on marketplace.json.

.claude-plugin/marketplace.json is kept as a fallback for direct-repo
installs; nothing in CI or scripts references it.

Refs mattpocock/personal-wiki#250

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

.agents/writing-docs.md

@@ -14,6 +14,8 @@ There is no H1 — the published page takes its title from the slug.
 
 Fill the template below. The **fixed frame** (Quickstart block, source link, `## What it does`, `## When to reach for it`, `## Where it fits`) appears on every page. The **adaptable middle** — `## Prerequisites` and the free-form substance sections — carries only what this particular skill earns; delete the rest.
 
+Install commands are not written per page. Copy them verbatim from [the canonical install block](./install-block.md) — it is the single source for how anyone installs these skills, and a page that words it differently is a page that tells a second story.
+
 <page-template>
 
 Quickstart: