docs(agent): let authorities coalesce last-seen writes

docs/agent-identity/INTENT.agent-identity.md

@@ -134,7 +134,8 @@ Not intended:
 ### Configuration and metadata
 
 - `last_seen_at` moves only on actions that prove current possession of the device key; an enrollment replay and a stream closure prove none.
-  Creating the identity initializes it; every accepted renew, confirm (including an already-current retry) and check-in, every authenticated `Hello`, and every later agent message on an authenticated stream sets it to the authority's current time.
+  Creating the identity initializes it; every accepted renew, confirm (including an already-current retry) and check-in, and every authenticated `Hello`, sets it to the authority's current time.
+  Later agent messages on an authenticated stream also set it, but an authority may write them at most once every 5 seconds per stream, so that a busy channel does not write for every message; `last_seen_at` then trails the latest message by at most 5 seconds and never moves back.
 - Check-in is only a fallback for configuration and renewal requests while no channel is working.
   Features that need server push require the agent channel.
 - An enrollment token gives an identity no config by default.