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/package.json

@@ -5,7 +5,7 @@
   "main": "main.js",
   "scripts": {
     "start": "electron .",
-    "dev": "electron . --dev",
+    "dev": "node scripts/dev.js",
     "dev:renderer": "vite renderer",
     "build:renderer": "vite build renderer",
     "build": "npm run build:renderer && electron-builder",