fix: preserve shared skill mounts across concurrent sandboxes

[eshulman2]

Summary

• mount shared skill directories with the shared SELinux z label
• retain read-only access while preventing concurrent containers from relabeling each other
• add a regression test for the mount mode

Cause

Skill directories used ro,Z, which assigns a private SELinux label. Concurrent sandbox starts can relabel the same host directory and cause an already-running container to fail with PermissionError while reading review.md.

Verification

• pytest -q tests/unit/sandbox/test_runner_mounts.py tests/unit/sandbox/test_podman_driver.py
• ruff check src/forge/sandbox/runner.py tests/unit/sandbox/test_runner_mounts.py
• git diff --check