M4 is split into M4a and M4b. The M4a spec rests on facts checked against
the owner's server and Mattermost's source at v11.11.0: the REST and
WebSocket shapes, and that clients will not post a message starting with
'/'. Records the design decisions, proposes P15 (secrets from a systemd
credential, the environment or an owner-only file), and adds the run-time
rows egress.md was missing since M2a and M3b.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
