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/types.ts

@@ -26,7 +26,7 @@ export interface MessagingGroupAgent {
   agent_group_id: string;
   trigger_rules: string | null; // JSON: { pattern, mentionOnly, excludeSenders, includeSenders }
   response_scope: 'all' | 'triggered' | 'allowlisted';
-  session_mode: 'shared' | 'per-thread';
+  session_mode: 'shared' | 'per-thread' | 'agent-shared';
   priority: number;
   created_at: string;
 }