refactor: flatten skills/ and adopt Agent Plugins 1.0

The Agent Plugins standard reads only the immediate children of
skills/ and cannot be pointed elsewhere, so a bucketed tree is
invisible to every conformant client.

Rather than generate a flat copy of the repo into a package
directory — a builder, a validator, an allowlist, a drift CI job,
and every skill committed twice — remove the buckets.

- skills/ is flat and holds exactly the promoted 25
- in-progress/ -> drafts/, misc/ -> extras/, deprecated/ deleted
- new root plugin.json (Agent Plugins 1.0)
- .claude-plugin/plugin.json loses its 25-entry skills array
- docs/ keeps its category folders; it is now the only place a
  skill's category lives

Reasoning in .agents/adr/0003.

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

.agents/writing-docs.md

@@ -1,10 +1,10 @@
 # Writing docs pages
 
-Every skill in `engineering/` and `productivity/` has a human-facing **docs page** at `docs/<bucket>/<skill-name>.md` — the docs tree mirrors those two bucket folders under `skills/`. It is published at `https://aihero.dev/skills-<skill-name>`; the URL is always `skills-<skill-name>` regardless of bucket, so the docs path is repo organisation only. The page is not the skill and not a copy of `SKILL.md`. Only these two buckets are promoted; the rest (`misc/`, `in-progress/`, `deprecated/`) ship no docs page.
+Every promoted skill — every folder under `skills/` — has a human-facing **docs page** at `docs/<category>/<skill-name>.md`, where `<category>` is `engineering` or `productivity`. The skills tree itself is flat, so this docs folder is the only place a skill's category lives. The page is published at `https://aihero.dev/skills-<skill-name>`; the URL is always `skills-<skill-name>` regardless of category, so the docs path is repo organisation only. The page is not the skill and not a copy of `SKILL.md`. Skills in `drafts/` and `extras/` ship no docs page.
 
 Most of these skills are **user-invoked**: the agent will never fire them for you, so *you* are the index that has to remember they exist and when to reach for them. That memory is **cognitive load**. The job of a docs page is to relieve it — to orient one reader around one skill so they can hold it in their head, know when to reach for it, and see where it sits in the system. The pages are collectively a distributed router; each is a node.
 
-Act whenever a promoted skill is added, renamed, or has its behaviour changed: create or re-sync its docs page. A rename moves the file too (`docs/<bucket>/<old>.md` → `docs/<bucket>/<new>.md`), because the published URL tracks the name; a skill that moves between `engineering/` and `productivity/` moves its docs file to the matching folder. Skills in `misc/`, `in-progress/`, and `deprecated/` get no page — none of those buckets is promoted. A skill moving *out* of one of them into `engineering/` or `productivity/` gains a page; one moving the other way loses it.
+Act whenever a promoted skill is added, renamed, or has its behaviour changed: create or re-sync its docs page. A rename moves the file too (`docs/<category>/<old>.md` → `docs/<category>/<new>.md`), because the published URL tracks the name; recategorising a skill moves its docs file to the other folder and changes nothing else. Skills in `drafts/` and `extras/` get no page. A skill promoted into `skills/` gains a page; one moved out of `skills/` loses it.
 
 Because these pages are published on `aihero.dev`, **every link is absolute** — never a repo-relative path. A link to another skill points at `https://aihero.dev/skills-<name>`; a link into the repo points at its full `https://github.com/mattpocock/skills/...` URL. A relative link that works in the repo breaks once published.