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>

plugins/pr-review-toolkit/.claude-plugin/plugin.json

@@ -0,0 +1,9 @@
+{
+  "name": "pr-review-toolkit",
+  "version": "1.0.0",
+  "description": "Comprehensive PR review agents specializing in comments, tests, error handling, type design, code quality, and code simplification",
+  "author": {
+    "name": "Daisy",
+    "email": "daisy@anthropic.com"
+  }
+}