bug(vm): per-sandbox Unix socket paths exceed macOS sun_path limit

User Story

As a developer running the VM compute driver on macOS, I want per-sandbox Unix socket paths to fit within the OS sun_path limit so that sandbox creation does not fail with a socket bind error.

Problem Statement

The VM driver constructs per-sandbox Unix socket paths by joining {state_dir}/sandboxes/{sandbox_id}/{socket_name}. Sandbox IDs are UUID v4 strings (36 chars), and state_dir depends on the user's home directory. On macOS, struct sockaddr_un.sun_path is 104 bytes (103 usable). With the default state_dir (~/.local/state/openshell/vm-driver), the resulting socket path reaches 107 bytes — 3 bytes over the limit — and bind() fails.

/Users/benoitf/.local/state/openshell/vm-driver/sandboxes/3eb2ad45-bead-4c2e-bd10-1a4a7f3a2721/control.sock
└─────────────────────────────────── 107 bytes ──────────────────────────────────────┘

Path length breakdown:

/Users/benoitf/.local/state/openshell/vm-driver   (47)
/sandboxes/                                        (11)
3eb2ad45-bead-4c2e-bd10-1a4a7f3a2721               (36)
/control.sock                                      (13)
                                            total = 107 bytes
                                     macOS limit = 104 bytes
                                       over by 3 bytes

Impact / Why This Matters

When this happens, VM sandbox creation fails at the control socket bind step. The error surfaces as a low-level socket error, not as a clear path-length diagnostic.

The current workaround is to manually configure a short state_dir (e.g. /tmp/os-vm). or rename control.sock for ctl.sock for my case (as I reduced 3 extra chars)
This is non-obvious, fragile, and breaks persistence expectations. Any user whose home directory path is longer than /Users/benoitf (15 chars) would be even further over the limit.

Linux is less affected (108-byte limit) but not immune with deep state directories or longer usernames.

Acceptance Criteria

• Per-sandbox Unix socket paths fit within 103 usable bytes on macOS for any reasonable state_dir
• The fix does not break sandbox state directory layout or persistence/restore
• Existing sandbox directories remain discoverable (sandbox ID must still be recoverable from the filesystem)
• The gateway compute-driver socket path (run/compute-driver.sock) remains unaffected

Reproduction Steps

1. On macOS, start the gateway with --compute-driver vm using the default state_dir (~/.local/state/openshell/vm-driver)
2. Run openshell sandbox create --name test --from ubuntu:24.04
3. Observe the sandbox creation fails at the control socket bind

Environment

• OpenShell: development build (current main branch)
• OS: macOS (Apple Silicon) — sun_path is 104 bytes
• Runtime: VM compute driver (libkrun)

Logs

Error: bind() failed for /Users/benoitf/.local/state/openshell/vm-driver/sandboxes/3eb2ad45-bead-4c2e-bd10-1a4a7f3a2721/control.sock

Start with the VM compute driver’s per-sandbox socket path construction and reproduce the macOS bind failure using the default state_dir. Trace how sandbox directories are created and restored, then verify that the revised paths stay within the macOS limit, preserve discovery and persistence, and leave run/compute-driver.sock unchanged.