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]

tests/unit/mcp_servers/test_do_server_circuit_breaker.py

@@ -39,12 +39,9 @@ _FULL_MANIFEST = {
         "evidence",
         "progress",
         "notify",
-        "open_session",
-        "link_session",
         "notify_list",
         "notify_get",
         "notify_ack",
-        "channels",
         "pr_update",
     ],
     "read_tools": ["Read", "Glob", "Grep"],