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/channel-auth.ts

@@ -10,7 +10,7 @@
 import fs from 'fs';
 import path from 'path';
 
-import { CHANNEL_AUTH_REGISTRY } from '../migrate-v1/shared.js';
+import { CHANNEL_AUTH_REGISTRY } from './shared.js';
 
 function parseEnv(filePath: string): Map<string, string> {
   const out = new Map<string, string>();