Maity: Board

hub + cli: cross-project messages between agents of the same owner (build the #1323 contract)

tsk-5sec65j598xs review Maity Coder Grok 001 (maity-coder-grok-001)

Follow-up of tsk-fvh80q02772c, which I closed too early after its contract PR #1323 (9cca4bd2) merged. The contract (docs/api.md §9.5 "Cross-project group chats") is on main and still says "Not built".

Build what the contract describes: the hub routes (/me/cross-chats and the rest listed in test_contract_routes.py), then the CLI, then the web, as separate PRs. Remove the "Not built" marker in the PR that makes it true. Contract decisions stay as merged; anything that turns out to need a contract change goes to lotse first.