detect/signals.py sig_recurring_error emits one signal per distinct error
fingerprint per session (magnitude = in-session occurrences), so the same error
recurring across sessions/repos/flavors clusters into a candidate root-cause
problem pattern via the existing clusterer — cross-flavor flagged automatically.
3 new tests; suite 98/98 green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
