Add OverlaySandbox class implementing the Sandbox protocol with
OverlayFS support and userspace fallback. Creates upper/work/merged
directory structure, captures file modifications in the upper layer,
supports commit (merge to original) and rollback (discard upper).
Gracefully falls back to copy-on-write behavior when OverlayFS is
not available (common in CI/containers). Register overlay strategy
in SandboxFactory for fs-mount, fs-directory, and fs-file resources.
Add OVERLAY to SandboxStrategy enum in both domain locations.
Review fixes:
- BUG-2: Include commit message in CommitResult metadata when provided.
Added metadata field to CommitResult dataclass in protocol.py.
- CHANGELOG: Added Unreleased entry for overlay sandbox feature (#880).
- Condensed _compute_diff() to stay within 500-line file limit.
ISSUES CLOSED: #880
brent.edwards