Operations / System 1 extension for Hub Core.

The versioned `helixforge.hub-extension` descriptor, manifest, validation, and
State Hub retirement boundary are documented in
[`docs/hub-extension.md`](docs/hub-extension.md).

`ops-hub` is the operational truth surface for environments, hosts, clusters,
services, endpoints, releases, backups, incidents, risks, runbooks, readiness
gates, and migration waves. Generic hub framework work stays in `hub-core`;
operations-specific extension code belongs here. Historical Haskell Inter-Hub is
`inter-hub-haskell` (retired 2026-07-08).