adr: Mattermost as another agent surface

adrs/mattermost.md

@@ -0,0 +1,26 @@
+# Mattermost as another agent surface
+
+We'd like Mattermost alongside Slack and the web UI.
+
+Mattermost is the self-hosted alternative to Slack: free, runs on your own servers,
+and it covers the orgs that can't or won't put their chat in someone else's cloud. We
+self-host everything we run, and a Mattermost bot is how our team would actually talk
+to QM.
+
+The integration surface is close to Slack's: a bot account authenticated with a
+personal access token, an outbound WebSocket for real-time events, and webhooks for
+messages. Like Slack's Socket Mode, there's no public URL, domain or TLS to set up —
+which matters even more for deployments on VPNs or air-gapped networks, where the web
+UI isn't always reachable either.
+
+A few points that came up thinking it through:
+
+- threads are sessions: when a thread is archived or deleted, the sessions bound to it
+  need to close with a reason;
+- Mattermost renders standard markdown, so most of the existing message formatting
+  should carry over as-is;
+- long messages should chunk on paragraph boundaries, never inside a code fence;
+- replies should mention whoever asked, with mentionable roles coming from config.
+
+Same shape as the Discord proposal: a separate service on the chassis, talking to core
+over the HTTP API the way web-ui does, keeps Mattermost opt-in behind a bot token.