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/teams/backend.md

@@ -2,13 +2,6 @@
 
 ## Team: `backend`
 
-## Your Channels
-- `#backend-cell` - Primary cell channel
-- `#dev-all` - Cross-cell developer discussions
-- `#qa-all` - Cross-cell QA discussions (if QA)
-- `#pm-all` - PM coordination (if PM)
-- `#doc-all` - Documentation discussions (if Documenter)
-
 ## Tech Stack
 - **Language**: Python
 - **Framework**: FastAPI