Provision and mount the spool WAL disk for producer instances

[Baishan]

Adds GCE startup/deploy plumbing to attach and mount a persistent disk for the spool producer's local WAL directory, and wires SPOOL_MODE/ instance-type substitutions through the staging deploy targets.

[depthfirst-app]

⚪ Severity: LOW

chmod 0777 makes the host-backed WAL directory writable by every local user or process. A process with a local foothold can replace or delete WAL segments, or plant symlinks before RotatingWal opens them, corrupting queued production logs or causing writes to attacker-selected host paths.
Helpful? Add 👍 / 👎

💡 Fix Suggestion

Suggestion: Replace chmod 0777 with chmod 0700 to restrict the WAL directory to the owning user (root, since this is a GCE startup script) only. World-writable and group-writable bits are unnecessary because the Docker container is launched with --privileged and runs as root, so it has full access to a root-owned 0700 directory via the bind-mount. This eliminates the risk of any other local user or process tampering with, replacing, or planting symlinks inside the WAL directory.

⚠️ Experimental Feature: This code suggestion is automatically generated. Please review carefully.

Suggested change

	chmod 0777 "${WAL_HOST_MOUNT}"
	chmod 0700 "${WAL_HOST_MOUNT}"