Add `docs/COMMAND_CONTROL_PLANE.md` describing the shared contract: stable
`<domain>.<verb>` ids, read-vs-write authority, persistence scope, exact
target selection, typed unavailability, and the bounded receipt. Update
`docs/FLEET.md` for the new verbs and the `/fleet status` vs `/fleet
workers` split, and record both entries in the changelog.
Refs #1888, #4022.
Hmbown