Codex session cache write is not atomic — a torn write loses the whole cache

[chhhee10]

writeCacheEntry() in lib/codex-sessions.ts does a read-modify-write with no atomic rename:

function writeCacheEntry(sessionId: string, path: string): void {
  try {
    mkdirSync(dirname(CACHE_PATH), { recursive: true });
    const cache = readCache();
    cache[sessionId] = path;
    writeFileSync(CACHE_PATH, JSON.stringify(cache), "utf-8");   // not atomic
  } catch {
    // Cache is best-effort
  }
}

The dashboard fans out Codex session lookups in parallel on the projects page, so these writes can interleave.

Why it matters

A lost entry isn't the problem — that's just a cache miss, and the next lookup rescans and repopulates it.

The problem is a torn file. If a write is interrupted partway, readCache() hits invalid JSON, throws, and the entire cache map is lost — not one entry. That turns a harmless race into a full cache wipe.

Where to fix

lib/codex-sessions.ts:

1. Write to a process-unique temp path — ${CACHE_PATH}.${process.pid}.tmp
2. renameSync() it into place (atomic on the same filesystem)
3. On failure, unlink the temp file so nothing is left behind

Keep the whole thing best-effort: the cache is an optimisation and must never throw into the caller.

Done when

• a test proves the cache file is written correctly and no .tmp files survive
• existing lib/codex-sessions tests still pass

_getCacheFilePath() is already exported from that module for exactly this kind of test.

Small and self-contained. Supersedes #276, which was closed while the fix was still unmerged, so it never landed.