feat: Bundle core plugins into claude-code repo

Add bundled plugins from claude-code-marketplace into a new plugins/ directory:
- agent-sdk-dev: Development kit for Claude Agent SDK with TypeScript/Python verifiers
- ireview-plugin: Comprehensive AI+Human PR review toolkit with 9 specialized agents
- commit-commands: Git commit workflow commands (commit, push, PR creation)

Created .claude-plugin/marketplace.json at repo root to register the bundled plugins.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -0,0 +1,40 @@
+{
+  "$schema": "https://anthropic.com/claude-code/marketplace.schema.json",
+  "name": "claude-code-plugins",
+  "version": "1.0.0",
+  "description": "Bundled plugins for Claude Code including Agent SDK development tools, PR review toolkit, and commit workflows",
+  "owner": {
+    "name": "Anthropic",
+    "email": "support@anthropic.com"
+  },
+  "plugins": [
+    {
+      "name": "agent-sdk-dev",
+      "description": "Development kit for working with the Claude Agent SDK",
+      "source": "./plugins/agent-sdk-dev",
+      "category": "development"
+    },
+    {
+      "name": "pr-review-toolkit",
+      "description": "Comprehensive PR review agents specializing in comments, tests, error handling, type design, code quality, and code simplification",
+      "version": "1.0.0",
+      "author": {
+        "name": "Anthropic",
+        "email": "support@anthropic.com"
+      },
+      "source": "./plugins/pr-review-toolkit",
+      "category": "productivity"
+    },
+    {
+      "name": "commit-commands",
+      "description": "Commands for git commit workflows including commit, push, and PR creation",
+      "version": "1.0.0",
+      "author": {
+        "name": "Anthropic",
+        "email": "support@anthropic.com"
+      },
+      "source": "./plugins/commit-commands",
+      "category": "productivity"
+    }
+  ]
+}