session-memory: recurring-error signal + clustering (WP-0006 T02)

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>

tests/test_detect_recurring_error.py

@@ -0,0 +1,59 @@
+"""Recurring-error signal + clustering (WP-0006 T02)."""
+
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+from session_memory.detect.cluster import cluster  # noqa: E402
+from session_memory.detect.signals import (  # noqa: E402
+    extract_signals,
+    sig_recurring_error,
+)
+
+
+def _digest(uid, repo, flavor="claude", snippets=None):
+    return {
+        "session_uid": uid, "flavor": flavor, "repo": repo, "outcome": "success",
+        "cost": {"input_tokens": 1, "output_tokens": 1},
+        "markers": {"errors": 0, "retries": 0, "test_runs": 0},
+        "tool_histogram": {}, "error_snippets": snippets or [],
+    }
+
+
+_FP = "modulenotfounderror: no module named 'foo' at <path>:<n>"
+
+
+def test_signal_per_distinct_fingerprint():
+    d = _digest("claude:a", "r1", snippets=[
+        {"fingerprint": _FP, "sample": "ModuleNotFoundError ...", "count": 3, "tool": "Bash"},
+        {"fingerprint": "keyerror: <str>", "sample": "KeyError", "count": 1, "tool": None},
+    ])
+    sigs = sig_recurring_error(d, {})
+    assert len(sigs) == 2
+    top = [s for s in sigs if s.locus == _FP][0]
+    assert top.type == "recurring_error"
+    assert top.magnitude == 3.0
+    assert top.detail["sample"].startswith("ModuleNotFound")
+
+
+def test_clusters_across_sessions_and_flavors():
+    # same fingerprint in a claude and a grok session -> cross-flavor candidate
+    digs = [
+        _digest("claude:a", "r1", "claude",
+                [{"fingerprint": _FP, "sample": "ModuleNotFoundError", "count": 2, "tool": "Bash"}]),
+        _digest("grok:b", "r2", "grok",
+                [{"fingerprint": _FP, "sample": "ModuleNotFoundError", "count": 1, "tool": None}]),
+    ]
+    signals = extract_signals(digs)
+    pats = cluster([s for s in signals if s.type == "recurring_error"], min_frequency=2)
+    assert len(pats) == 1
+    p = pats[0]
+    assert p.signal_type == "recurring_error"
+    assert p.cross_flavor is True
+    assert sorted(p.flavors) == ["claude", "grok"]
+    assert p.frequency == 2
+
+
+def test_no_snippets_no_signal():
+    assert sig_recurring_error(_digest("claude:a", "r1"), {}) == []