Fix observe.observed not emitting for tmux-only segments
Segments with only .jsonl files (like tmux captures) weren't triggering
segment insights because no handlers matched, so observe.observed was
never emitted.
- Add immediate observe.observed emission when no files match handler
patterns in _handle_callosum_message()
- Extract _emit_segment_observed() helper to consolidate duplicated
emit + cleanup logic
- Add test_file_sensor_segment_observed_no_handlers test
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>