## Description
Headroom can launch its MCP server, but did not publish a canonical
`server.json` that registries and MCP hosts can consume directly. This
PR adds a shared descriptor builder, commits a root `server.json`,
parity-tests that artifact against the builder and existing runtime
spec, and updates docs so registry authors do not need to reconstruct
`headroom mcp serve` from prose.
Closes#929.
## Type of Change
- [ ] Bug fix (non-breaking change that fixes an issue)
- [x] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update
- [ ] Performance improvement
- [ ] Code refactoring (no functional changes)
## Changes Made
- Added a shared `server_json.py` descriptor builder for Headroom MCP
publication metadata.
- Published a canonical root `server.json` and parity-tested it against
the builder.
- Encoded the publishable uvx contract as `headroom-ai[mcp]` plus
`headroom mcp serve`.
- Updated README and MCP docs to point registry authors at the canonical
descriptor.
- Added the README ownership marker used by MCP Registry verification.
- Kept existing registrars and `headroom mcp install` behavior
unchanged.
## Testing
- [x] Unit tests pass
- [x] Linting passes
- [x] Type checking passes
- [x] New tests added for new functionality when applicable
- [x] Manual testing performed
### Test Output
```text
Focused registry/server-json tests and reviewer approval were completed on this PR before the governance body cleanup. The current body update is documentation-only metadata for PR governance.
```
## Real Behavior Proof
- Environment: Headroom development checkout with MCP test dependencies.
- Exact command / steps: Inspected the generated `server.json` contract
and parity coverage against the descriptor builder and runtime MCP spec.
- Observed result: The committed descriptor matches the builder/runtime
contract and advertises the intended `headroom-ai[mcp]` / `headroom mcp
serve` launch path.
- Not tested: live publication to third-party registries
## Review Readiness
- [x] I have performed a self-review
- [x] This PR is ready for human review
## Additional Notes
This body was normalized by a maintainer after approval so the
governance parser reflects the already-reviewed PR state.
