Fix negative durations from clock-skewed transcripts

[Gambit-Checkmate]

Clock-skewed transcripts could produce negative run durations and reduce the total agent time. Treat an end timestamp earlier than the start as an unknown duration (None), which the CLI already excludes from duration statistics.

The regression test checks that durations [-5, 10, 20] total 30 seconds with a median of 15 seconds. It failed before the fix (total 25 seconds). All 29 tests pass; the updated test file passes Ruff and git diff --check passes.

Closes #22.

[github-actions]

This is in, @Gambit-Checkmate. Really appreciate it.

Good, focused change. Exactly the kind of thing Agentrace needs.

If Agentrace is useful to you, a star goes a long way.

Happy to see more from you whenever you have the time.