chore(comms): finalize #306 teardown — changelog + purge dangling refs

#306 removed the channels/sessions/messages subsystem but left dangling
references. Agents were still told to call removed verbs at spawn, and a
maintenance script referenced a dropped table.

- prompts (roles/identities/teams/base): drop say/open_session/link_session/
  channels() and the dead Channels sections; comms rows now teach A2A
  (dm + read_a2a); renumber the PM workflow steps the removed open_session
  step left behind
- scripts/reset_runtime_state.sql: drop the dropped chunks_conversations table
- models/events.py: mark the retired SESSION_*/MESSAGE_SENT enum members inert
- mcp/{do,flow}_server.py: drop the dead SESSION_CLOSED error-map key
- panel: drop channels_read/write from AgentPermissions; remove dead channel:
  KB source branch
- pyproject.toml: refresh a ruff-exemption example off the removed verb kwargs
- CHANGELOG: record #306 under [Unreleased]

roboco/services/a2a.py

@@ -1835,8 +1835,8 @@ class A2AService:
     ) -> None:
         """Best-effort publish of A2A_MESSAGE_SENT for the operator live view.
 
-        Mirrors MessagingService.send_message's publish pattern: a bus outage
-        is logged and never rolls back the already-persisted message.
+        A bus outage is logged and never rolls back the already-persisted
+        message.
         """
         try:
             bus = get_event_bus()