[Linux] Reserve missing top level .git at runtime

[evawong-oai]

Summary

A writable workspace with no .git at startup could still run git init inside Linux bubblewrap, bypassing sandbox filesystem permission controls. This change closes only that path.

Link - https://tracker.bugcrowd.com/openai/security-inbox/submissions/f5e108eb-aec3-4031-a622-00dd5783889f?utm_campaign=deprecatedroutes&utm_source=bugcrowd&sortBy=submittedAt&sortDirection=asc

Root Cause

The protected subpath policy needs to include top level .git even when it does not exist yet.

The Linux bubblewrap runtime also needs to reserve that missing path at execution time.

Without both parts, git init can create .git later inside the sandbox.

Scope

This PR touches six files because the fix spans the shared protected path policy and the Linux runtime that enforces it.

1. protocol.rs and permissions.rs ensure top level .git is treated as protected whether it exists or not.
2. bwrap.rs reserves a missing top level .git with the existing /dev/null path and tracks only that synthetic mount point for cleanup.
3. linux_run_main.rs waits only when that cleanup path is needed, then removes only the synthetic empty .git placeholder.
4. landlock.rs adds the Linux regression test for the reported path.
5. seatbelt_tests.rs updates the shared policy expectations that now include top level .git in protected paths.