Reject implicitly discovered bare Git repositories (#36924)

## Why

A repository can contain a tracked directory that Git implicitly treats as a bare
repository. Its configuration may select helpers such as `core.fsmonitor`, causing
Codex Git operations in that directory to execute repository-controlled code.

## What changed

- Pass `-c safe.bareRepository=explicit` to Codex-managed Git commands so they
  reject implicitly discovered bare repositories.
- Continue to support repositories explicitly selected with `--git-dir` or
  `GIT_DIR`.

## Testing

Add a regression test that clones a repository containing a tracked embedded Git
repository and verifies that guarded Git inspection rejects it without running
its configured filesystem monitor.

GitOrigin-RevId: 344b5bc1e0ffa94f2a1b788488aa653222da10f3

codex-rs/core-plugins/src/loader.rs

@@ -1675,7 +1675,9 @@ fn run_git(args: &[&str], cwd: Option<&Path>) -> Result<(), String> {
 
 fn run_git_output(args: &[&str], cwd: Option<&Path>) -> Result<String, String> {
     let mut command = Command::new("git");
-    command.args(args);
+    command
+        .args(["-c", codex_git_utils::SAFE_BARE_REPOSITORY_CONFIG])
+        .args(args);
     command.env("GIT_TERMINAL_PROMPT", "0");
     if let Some(cwd) = cwd {
         command.current_dir(cwd);