chore: move shared helpers to migrate-v2/, delete migrate-v1/

Extracted the helpers we use (JID parsing, trigger mapping, channel
auth registry, generateId, v2PlatformId) into setup/migrate-v2/shared.ts.
Deleted setup/migrate-v1/ entirely — no code references it anymore.

Updated README, CLAUDE.md, docs/v1-to-v2-changes.md, and
docs/migration-dev.md to reference the new paths and migrate-v2.sh
entry point.

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

setup/migrate-v2/db.ts

@@ -29,8 +29,7 @@ import {
   generateId,
   parseJid,
   triggerToEngage,
-  JID_PREFIX_TO_CHANNEL,
-} from '../migrate-v1/shared.js';
+} from './shared.js';
 
 interface V1Group {
   jid: string;