main: keep mount parent dirs runner-owned in fallback path

[piob-io]

Summary

Fixes root-owned parent directories left behind when a sticky disk fails to expose, which surfaces later as EACCES errors from tools like corepack (Error: EACCES: permission denied, mkdir '/home/runner/.cache/node/corepack/v1').

Root cause: on expose timeout/failure, ensureFallbackDirectory ran only sudo mkdir -p <path>, so any missing intermediate dirs (e.g. /home/runner/.cache for a ~/.cache/uv mount) were created root:root, and only the leaf was chowned back to the runner. The successful mount path (mountStickyDisk) already avoided this by first running a non-sudo mkdir -p on the parent.

Change: extract that logic into createMountPoint(stickyDiskPath) (non-sudo parent mkdir -p → sudo mkdir -p leaf → chown leaf + workspace-local parent) and use it from both mountStickyDisk and ensureFallbackDirectory.

Observed in production on blacksmith-01kwz990dntbn1dvjhk60tyzmb-4vcpu (langchain-ai): three sticky disks (go-cache, pnpm-store, uv-cache) hit the 45s expose timeout, the fallback created /home/runner/.cache as root, and the next step's corepack invocation failed with EACCES.

Link to Devin session: https://app.devin.ai/sessions/c8e9c0baeeae4bb2b08f6327a3bf477f
Requested by: @piob-io

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