## Why
The Linux sandbox represents missing protected metadata paths such as `.git`
with empty read-only directories. Repository and project discovery treated any
`.git` directory as a checkout root, so this synthetic directory could hide a
real parent repository and its trust configuration.
## What changed
- Treat a `.git` directory as repository metadata only when it contains
`HEAD`, while continuing to recognize file-based `.git` entries.
- Protect missing `.git` paths with the same read-only synthetic mounts as
other workspace metadata without disrupting parent-repository discovery.
- Canonicalize and read-only bind the synthetic-mount registry into the
sandbox, and make protected-path cleanup handle read-only directory trees.
## Testing
Added coverage for repository, project-root, trust, synthetic-mount, and
Landlock behavior, including nested incomplete `.git` directories and a
redirected `TMPDIR`.
GitOrigin-RevId: 0724c54d56531143bb28011e4ca414cd3b0212be
Eric Traut
9ca99b5171