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]

pyproject.toml

@@ -151,7 +151,7 @@ select = [
 "roboco/mcp/**/*.py" = ["PLC0415", "PLR0913"]
 "roboco/services/*.py" = ["PLC0415"]
 # Gateway methods are typed verb surfaces — agent-facing kwargs reflect the
-# verb contract (session topic, channel, relationship type, etc.). Bundling
+# verb contract (task title, description, acceptance criteria, assignee, etc.). Bundling
 # into a dataclass hides the field-by-field schema the LLM needs at the
 # tool layer; we accept the >5 kwarg signatures here for the same reason
 # they're accepted in `roboco/mcp/**`.