Add the operator-requested M9 after M8, delivering native A2A over Habitat in the new, separate habitat-a2a repository. The plan defines a pinned SDK/binding proof, a usable durable-task package and executor interface, and real-agent recovery/isolation/release gates. Messenger changes are limited to the necessary reusable delivery API; Git A2A and public HTTP remain later work.
This is a documentation-only planning PR; no implementation or tests are added. Existing M6-M8 gates and human merge policy are preserved. M9 implementation requires independent exact-tip plan satisfaction and M8 closure.
Validation: git diff --check; checked phase order and relative plan links. Independent plan review pending.