refactor: migrate plugin structure from .claude-plugin to project root

Restructure plugin to follow Claude Code official documentation:
- Move TypeScript files from .claude-plugin/* to project root
- Create Markdown command files in commands/
- Update plugin.json to reference ./commands/*.md
- Add comprehensive plugin installation guide

Changes:
- Commands: pm.md, research.md, index-repo.md (new Markdown format)
- TypeScript: pm/, research/, index/ moved to root
- Hooks: hooks/hooks.json moved to root
- Documentation: PLUGIN_INSTALL.md, updated CLAUDE.md, Makefile

Note: This commit represents transition state. Original TypeScript-based
execution system was replaced with Markdown commands. Further redesign
needed to properly integrate Skills and Hooks per official docs.

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

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

hooks/hooks.json

@@ -0,0 +1,15 @@
+{
+  "hooks": {
+    "SessionStart": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "/pm",
+            "timeout": 30
+          }
+        ]
+      }
+    ]
+  }
+}