feat: create fallback directory on sticky disk mount failure

[devin-ai-integration]

Summary

When the sticky disk mount fails (timeout, ConnectError: [canceled], etc.), the target directory is never created because mountStickyDisk bails before the mkdir -p / mount steps. This causes downstream workflow steps to crash on a missing path — the job fails entirely instead of gracefully degrading to a cold cache.

This adds a fallback after mount failure:

if (stickyDiskError) {
  core.warning(`Error getting sticky disk: ${stickyDiskError}`);
  // Create empty dir so subsequent steps don't fail
  await execAsync(`sudo mkdir -p ${shellQuote(stickyDiskPath)}`);
  await execAsync(`sudo chown $(id -u):$(id -g) ${shellQuote(stickyDiskPath)}`);
}

Jobs now continue with an empty directory (cold cache) instead of failing. Triggered by Pylon #8047 — twin-so hitting ~31% failure rate due to us-west Ceph cluster degradation.

Link to Devin session: https://app.devin.ai/sessions/8950e9fa29ce4e1098552fa904fd6109

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