Introduces a standardized way for registries to provide experimental or
community-driven features without committing them to the core API
specification.
## Summary
- Extensions live under `/v0/x/<namespace>/<extension>` URL structure
- Simple conventions: REST, JSON responses, cursor pagination, optional
auth
- Composable design building on core APIs where possible
This enables experimentation with features like search (#389) and MCP
server integration (#24) while keeping the core API minimal and stable.
Fixes#776
