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>

docs/v2-checklist.md

@@ -55,8 +55,10 @@ Status: [x] done, [~] partial, [ ] not started
 - [~] iMessage via Chat SDK (adapter + skill written, not tested)
 - [x] Backward compatibility with native channels (old adapters still work)
 - [x] Channel barrel wired (src/index.ts imports barrel, skills uncomment)
-- [~] Setup flow wired to v2 channels (register.ts + verify.ts updated, but channel skills don't call register yet — see docs/v2-setup-wiring.md)
-- [ ] Setup communicates each group is a different agent, distinct names
+- [x] Setup flow wired to v2 channels (channel skills + /manage-channels for registration + verify.ts checks all tokens)
+- [x] Channel Info metadata in each channel skill (type, terminology, how-to-find-id, isolation defaults)
+- [x] /manage-channels skill (wire channels to agent groups with three isolation levels)
+- [x] Agent-shared session mode (cross-channel shared sessions, e.g. GitHub + Slack)
 - [ ] Setup vs production channel separation
 - [ ] Generate visual diagram of customized instance at end of setup