core: _gitexec trusted-executable path validation untested (coverage-003)

[SUaDtL]

Branch: feat/pi-support · Tribunal run 2026-07-17-pi-support-branch · Finding: findings/coverage/coverage-003.json
core/pysrc/_gitexec.py:16-37 _trusted_environment_path has no direct unit test.

[SUaDtL]

Verified against PR #313 head 9ed248f4: plugins/ca/hooks/tests/test_gitexec.py directly exercises _gitexec._trusted_environment_path for missing, relative, nonexistent, non-file, non-executable, and valid executable paths. The cross-host hook matrix is green on Linux, macOS, and Windows. This finding is implemented on #313, which now carries Closes #322.