Add Claude Code plugin manifest and marketplace catalog

Enable this repo to work as a Claude Code plugin so users can
install it via `/plugin marketplace add addyosmani/agent-skills`.

Add:
- .claude-plugin/plugin.json — plugin manifest exposing
  .claude/commands/ as namespaced slash commands under
  the agent-skills: prefix
- .claude-plugin/marketplace.json — marketplace catalog
  listing this repo as the agent-skills plugin

Fix:
- hooks/hooks.json — rewrite from deprecated flat-array format
  to the plugin-system format keyed by event type; use
  ${CLAUDE_PLUGIN_ROOT} so the hook resolves correctly when the
  plugin is installed from the marketplace into the local cache
- README — replace non-existent `claude plugin add` with the
  correct `/plugin marketplace add` workflow; document namespaced
  slash commands and --plugin-dir usage; add .claude-plugin/ to
  the project tree

.claude-plugin/marketplace.json

@@ -0,0 +1,19 @@
+{
+  "name": "addy-agent-skills",
+  "owner": {
+    "name": "Addy Osmani"
+  },
+  "metadata": {
+    "description": "Production-grade engineering skills for AI coding agents — covering the full software development lifecycle from spec to ship."
+  },
+  "plugins": [
+    {
+      "name": "agent-skills",
+      "source": {
+        "source": "github",
+        "repo": "addyosmani/agent-skills"
+      },
+      "description": "Production-grade engineering skills covering every phase of software development: spec, plan, build, verify, review, and ship."
+    }
+  ]
+}

.claude-plugin/plugin.json

@@ -0,0 +1,12 @@
+{
+  "name": "agent-skills",
+  "description": "Production-grade engineering skills for AI coding agents — covering the full software development lifecycle from spec to ship.",
+  "version": "1.0.0",
+  "author": {
+    "name": "Addy Osmani"
+  },
+  "homepage": "https://github.com/addyosmani/agent-skills",
+  "repository": "https://github.com/addyosmani/agent-skills",
+  "license": "MIT",
+  "commands": "./.claude/commands"
+}