fix(migrate-v2): correct JID parsing, Discord guildId lookup, silent failures

- shared.ts: parseJid now recognizes raw Baileys WhatsApp JIDs
  (`<id>@s.whatsapp.net`, `@g.us`, etc.); v2PlatformId returns the raw
  JID for whatsapp to match what the runtime adapter emits. Without this,
  every WhatsApp group in a v1 install was silently skipped.

- discord-resolver.ts: new helper that uses DISCORD_BOT_TOKEN to look up
  channelId → guildId via the Discord API, since v1 stored only the
  channel id but v2 needs `discord:<guildId>:<channelId>`. Best-effort:
  on missing/invalid token or network error, returns empty resolver and
  the affected groups are skipped with the reason surfaced per channel.

- db.ts, tasks.ts: route Discord groups through the resolver; other
  channels go through v2PlatformId unchanged. Resolver only built when
  at least one Discord group exists, so non-Discord installs incur no
  network.

- db.ts: when every v1 group is skipped, exit non-zero with a FAIL line
  instead of `OK:groups=N,...,skipped=N`, so the wrapper doesn't hide
  total failure under a successful-looking summary.

- migrate-v2.sh: run_step now surfaces ERROR: lines from successful
  steps (with count + first 3 + raw log path); phase 2c install loop
  populates STEP_RESULTS so install failures show in handoff.json
  instead of silently passing.

- sessions.ts: copyTree skips dangling symlinks (e.g. v1's
  `.claude/debug/latest`) instead of crashing the entire step.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

setup/migrate-v2/sessions.ts

@@ -50,6 +50,8 @@ function copyTree(src: string, dst: string): number {
       written += copyTree(s, d);
       continue;
     }
+    // Skip dangling symlinks (e.g. v1's .claude/debug/latest pointer).
+    if (entry.isSymbolicLink() && !fs.existsSync(s)) continue;
     if (fs.existsSync(d)) continue;
     fs.copyFileSync(s, d);
     written += 1;