Discovery rules:
1. extensions/*.ts or *.js - direct files
2. extensions/*/index.ts or index.js - subdirectory with index
3. extensions/*/package.json with pi field - load declared paths
No recursion beyond one level. Complex packages use package.json manifest.
Added PiManifest type for future theme/skill bundling support.
17 tests covering all discovery scenarios.
refs #454
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
