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]

panel/src/components/knowledge-base/kb-result-card.tsx

@@ -20,9 +20,6 @@ export function KBResultCard({ result, onClick }: KBResultCardProps) {
   // Format source for display
   const formatSource = (source: string) => {
     // Remove common prefixes
-    if (source.startsWith("channel:")) {
-      return source.replace("channel:", "# ");
-    }
     if (source.startsWith("journal:")) {
       return source.replace("journal:", "Journal: ");
     }

panel/src/types/index.ts

@@ -360,8 +360,6 @@ export interface ModelConfig {
 
 export interface AgentPermissions {
   can_notify: boolean;
-  channels_read: string[];
-  channels_write: string[];
 }
 
 export interface AgentMetrics {