address review: add thread resolution test, log catch, remove stray comment

- Add integration test for per-destination thread_id resolution: seeds two
  destinations with different thread IDs, verifies each outbound message
  carries the correct thread_id (not a global one from the batch routing).
- Add log line in resolveDestinationThread catch block for debuggability.
- Remove stray "(ensurePreCompactHook is defined after the main function.)"
  comment from group-init.ts.

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

src/group-init.ts

@@ -89,7 +89,6 @@ export function initGroupFilesystem(group: AgentGroup, opts?: { instructions?: s
 
   // Skills directory — created empty here; symlinks are synced at spawn
   // time by container-runner.ts based on container.json skills selection.
-  // (ensurePreCompactHook is defined after the main function.)
   const skillsDst = path.join(claudeDir, 'skills');
   if (!fs.existsSync(skillsDst)) {
     fs.mkdirSync(skillsDst, { recursive: true });