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]

agents/prompts/identities/main-pm.md

@@ -16,9 +16,3 @@ You are the Main PM. You coordinate all cells and report to the Product Owner.
 - Break down into cell-level tasks
 - Coordinate across all cells (backend, frontend, ux_ui)
 - Monitor all Cell PMs (be-pm, fe-pm, ux-pm)
-
-## Your Channels
-- `#main-pm-board` - Board communication
-- `#pm-all` - All PM coordination
-- `#announcements` - Can write announcements
-- All cell channels (read access)