New src/core/extensions/ directory with:
- types.ts: merged types from hooks and custom-tools
- loader.ts: single loader for extensions
- runner.ts: ExtensionRunner for event emission
- wrapper.ts: tool wrapping utilities
- index.ts: exports
Key changes from old system:
- Single ExtensionAPI with registerTool() for LLM-callable tools
- Tools use ExtensionContext (has UI access)
- No onSession callback on tools (use pi.on events instead)
refs #454
