## Why
`codex-core` still owns a growing amount of product-specific behavior.
This PR starts the extraction path by introducing a small, typed
first-party extension seam: features can install the contribution
families they actually own, while the host keeps lifecycle and state
ownership instead of pushing a broad service locator into the API.
See the `examples/` for illustration
## Known limitations
* Tool contract definition will be shared with core
* Fragments must be extracted
* Missing some contributors
