Part of #222. Implement the on-disk registry that tracks installed extensions. - Module at `src/extension/store.ts` with `readExtensions`, `writeExtensions`, `findExtension` exports - Registry persisted as JSON at `~/.elastic/extensions.json` - Each entry: `{ name, source, path, entrypoint }` - Unit tests in `test/extension/store.test.ts`
Part of #222.
Implement the on-disk registry that tracks installed extensions.
src/extension/store.tswithreadExtensions,writeExtensions,findExtensionexports~/.elastic/extensions.json{ name, source, path, entrypoint }test/extension/store.test.ts