v2: channel isolation model, manage-channels skill, refactored channel skills

- Add three-level isolation model (shared session, same agent, separate agent)
  with agent-shared session mode for cross-channel shared sessions
- Create /manage-channels skill for wiring channels to agent groups
- Refactor all 12 v2 channel skills: lean SKILL.md + VERIFY.md + REMOVE.md
  with structured Channel Info section for platform-specific metadata
- Create /add-discord-v2 skill (was missing)
- Add step 5a to setup SKILL.md invoking /manage-channels after channel install
- Update setup/verify.ts to check all 12 channel token types
- Add docs/v2-isolation-model.md explaining the isolation model
- Update v2-checklist.md and v2-setup-wiring.md to reflect completed work

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/db/sessions.ts

@@ -27,6 +27,13 @@ export function findSession(messagingGroupId: string, threadId: string | null):
     .get(messagingGroupId, 'active') as Session | undefined;
 }
 
+/** Find an active session scoped to an agent group (ignoring messaging group). */
+export function findSessionByAgentGroup(agentGroupId: string): Session | undefined {
+  return getDb()
+    .prepare("SELECT * FROM sessions WHERE agent_group_id = ? AND status = 'active' ORDER BY created_at DESC LIMIT 1")
+    .get(agentGroupId) as Session | undefined;
+}
+
 export function getSessionsByAgentGroup(agentGroupId: string): Session[] {
   return getDb().prepare('SELECT * FROM sessions WHERE agent_group_id = ?').all(agentGroupId) as Session[];
 }