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"
+}

README.md

@@ -23,26 +23,29 @@ AI coding agents are powerful, but they default to taking the shortest path —
 
 ### Claude Code (Recommended)
 
+**Via marketplace** — add and install directly from Claude Code:
+
+```
+/plugin marketplace add addyosmani/agent-skills
+/plugin install agent-skills@addy-agent-skills
+```
+
+**Local / development use** — clone and load with `--plugin-dir`:
+
 ```bash
-# Install as a Claude Code plugin
-claude plugin add agent-skills
-
-# Or clone and reference directly
 git clone https://github.com/addyosmani/agent-skills.git
-cd your-project
-# Add to your CLAUDE.md:
-# Import skills from /path/to/agent-skills/skills/
+claude --plugin-dir /path/to/agent-skills
 ```
 
-Once installed, skills are available via slash commands:
+Once loaded, skills activate automatically based on context. Slash commands are also available under the `agent-skills:` namespace:
 
 ```
-/spec      → Start spec-driven development
-/plan      → Break work into tasks
-/build     → Implement incrementally
-/test      → Run TDD workflow
-/review    → Trigger code review
-/ship      → Pre-launch checklist
+/agent-skills:spec      → Start spec-driven development
+/agent-skills:plan      → Break work into tasks
+/agent-skills:build     → Implement incrementally
+/agent-skills:test      → Run TDD workflow
+/agent-skills:review    → Trigger code review
+/agent-skills:ship      → Pre-launch checklist
 ```
 
 ### Cursor
@@ -182,8 +185,11 @@ agent-skills/
 ├── AGENTS.md                          # Guidance for AI agents
 ├── CLAUDE.md                          # Claude Code configuration
 ├── LICENSE                            # MIT License
+├── .claude-plugin/
+│   ├── plugin.json                    # Claude Code plugin manifest
+│   └── marketplace.json               # Claude Code marketplace catalog
 ├── .claude/
-│   └── commands/                      # Slash commands
+│   └── commands/                      # Slash commands (standalone use)
 ├── skills/                            # Core skills (SKILL.md per directory)
 │   ├── idea-refine/                   # Define phase: Idea refinement
 │   ├── using-agent-skills/            # Meta: how to use this pack

hooks/hooks.json

@@ -1,8 +1,14 @@
 {
-  "hooks": [
-    {
-      "type": "SessionStart",
-      "command": "bash hooks/session-start.sh"
-    }
-  ]
+  "hooks": {
+    "SessionStart": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "bash ${CLAUDE_PLUGIN_ROOT}/hooks/session-start.sh"
+          }
+        ]
+      }
+    ]
+  }
 }