tests/clean_test.py hangs in git mktree when stdin is an open socket #46

Seen while building #45 (not caused by it): on main, tests/clean_test.py hangs in git mktree when the test runner's stdin is an open socket (an agent harness); with < /dev/null the tests pass. The test (or the kit's clean code when it shells out) should not inherit stdin: pass stdin=subprocess.DEVNULL to the git calls that do not read stdin, and feed git mktree its input explicitly. Add a test that runs the suite with a socket on stdin, or at least a timeout on each git call.