fix(vm): relocate per-sandbox Unix sockets to /tmp to fit macOS sun_path

[benoitf]

Summary

Move VM driver per-sandbox Unix sockets (control.sock, ssh.sock) from the
state directory to /tmp/openshell/sandboxes/{sandbox-id}/ so that socket paths
stay within the 104-byte macOS sun_path limit regardless of home directory depth.

Related Issue

Closes #3452

Changes

• Add sandbox_socket_dir() helper returning /tmp/openshell/sandboxes/{sandbox-id}
  (hardcoded /tmp because macOS TMPDIR resolves to ~51-char /var/folders/…/T/
  which would re-exceed the limit)
• /tmp/openshell/sandboxes/ created with 0o1777 (sticky, multi-user safe);
  per-sandbox dirs remain 0o700
• control.sock and ssh.sock paths updated to use the new socket directory
• Socket directory cleanup added to all teardown paths: create_sandbox early
  errors, provision_sandbox cancellation, fail_provisioning, and delete_sandbox
• Socket dir also created in provision_sandbox_inner for the restoration path

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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