feat: index Codex sessions alongside Claude Code with source tagging

Add `source` column to sessions and messages ('claude' | 'codex').
  Discover and parse Codex JSONL files from ~/.codex/sessions/, mapping
  Codex thread/item structures to the same schema (messages, tool_calls,
  tool_results, subagents). Move DB to ~/.obelisk/ with legacy migration.
  Add rebuild-to-temp-then-swap for safe full rebuilds. On the app side:
  source filter toggle, collapsible untitled session fold, configurable
  codexDir in Settings, and a dev script. Update SKILL.md and query
  helpers to expose source fields and accept source filter opt.

app/indexer-worker-client.js

@@ -49,8 +49,9 @@ function createWorkerBuildIndex({
   const stop = () => {
     const current = worker;
     worker = null;
-    if (current?.terminate) current.terminate();
+    const termination = current?.terminate ? Promise.resolve(current.terminate()) : Promise.resolve();
     rejectPending(new Error('Indexer worker stopped'));
+    return termination;
   };
 
   return { buildIndex, stop };