feat: ship the skill set as a native Claude Code plugin

Add a Claude Code plugin marketplace so the promoted skills can be
installed as a managed, read-only bundle (subscribe rather than fork),
alongside the existing skills.sh installer.

- Enrich .claude-plugin/plugin.json with marketplace metadata (version,
  description, author, license, keywords); keep the curated promoted-only
  skills array.
- Add .claude-plugin/marketplace.json making the repo its own
  single-plugin marketplace (mattpocock-skills@mattpocock).
- README: document /plugin install alongside skills.sh, and the
  subscribe-vs-fork tradeoff.
- CLAUDE.md: extend the promoted-set invariant to cover marketplace.json
  and plugin.json/package.json version sync.
- ADR 0002: record why Claude ships now and a native Codex plugin is
  deferred (Codex skills field is single-path + drops symlinks, which
  can't express a curated subset of a bucketed repo without a restructure).

Verified: 'claude plugin validate . --strict' passes, and marketplace
add -> install resolves all 21 promoted skills at v1.2.0.

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

.claude-plugin/marketplace.json

@@ -0,0 +1,23 @@
+{
+  "name": "mattpocock",
+  "owner": {
+    "name": "Matt Pocock",
+    "url": "https://www.aihero.dev"
+  },
+  "description": "Matt Pocock's skills for real engineering, as an installable Claude Code plugin.",
+  "plugins": [
+    {
+      "name": "mattpocock-skills",
+      "source": "./",
+      "description": "Matt Pocock's agent skills for real engineering — grilling, spec/ticket flows, TDD, code review, domain modelling and more.",
+      "category": "engineering",
+      "keywords": [
+        "engineering",
+        "skills",
+        "tdd",
+        "code-review",
+        "grilling"
+      ]
+    }
+  ]
+}

.claude-plugin/plugin.json

@@ -1,5 +1,23 @@
 {
   "name": "mattpocock-skills",
+  "version": "1.2.0",
+  "description": "Matt Pocock's agent skills for real engineering — grilling, spec/ticket flows, TDD, code review, domain modelling and more. Plug-and-play, not vibe coding.",
+  "author": {
+    "name": "Matt Pocock",
+    "url": "https://www.aihero.dev"
+  },
+  "homepage": "https://www.aihero.dev/s/skills-newsletter",
+  "repository": "https://github.com/mattpocock/skills",
+  "license": "MIT",
+  "keywords": [
+    "engineering",
+    "skills",
+    "tdd",
+    "code-review",
+    "grilling",
+    "domain-modeling",
+    "productivity"
+  ],
   "skills": [
     "./skills/engineering/ask-matt",
     "./skills/engineering/diagnosing-bugs",