Daemon suite flakes on ubuntu CI: GitRepo fixture state vanishes mid-test #707

The ubuntu Build and test leg intermittently fails one daemon-suite test whose fixture is a GitRepo, with git state that should be stable disappearing or mutating between two steps of the same test. Two recent occurrences, different tests, same shape:

• Run 33260948585 (main, 53be3b75): Borrowed_launch_creates_no_worktree_and_runs_in_the_cwd — the before/after tree snapshots differ by exactly .git/objects/maintenance.lock, a transient artifact of git's background maintenance, which evidently runs inside the fixture repos on CI.
• Run 33397578952 (PR Block chat messaging to a flow participant in the desktop app #706, App-only diff): Interactive_codex_launch_threads_the_posture_into_the_runtime_start_context — GitRepo.CreateWithCommit fails 10 ms in with git add -A → 128 fatal: not a git repository, immediately after a successful git init in a uniquely named owned TempDir (Directory.CreateTempSubdirectory, so a path collision is ruled out).

Both runs' suite stderr also carries UnixSpawnerThread terminated unexpectedly / Environment.FailFast — possibly the intentional crash-contract test's child process rather than part of the flake; not yet confirmed either way.

The maintenance.lock sighting suggests the assembly-wide git config pin (Guards/GitConfigGlobalSetup) does not disable background maintenance/auto-gc in fixture repos; a detached git maintenance process from an earlier test also outlives its TempDir. Whether that also explains the not a git repository case is open.