session-memory Phase 1: Detect pipeline (T04-T07)

- detect/signals.py: pure extractors over digests (retry storm, repeated
  errors, budget overrun vs corpus p90, abandoned, clean pass, recovery)
- detect/cluster.py: deterministic clustering into candidate Patterns with
  evidence (sessions/repos/flavors/cost impact) + cross-flavor flagging
- detect/__main__.py: python -m session_memory.detect, ranked report
  (cross-flavor first) + --json; persists candidates to Tier 2 patterns table
- core/store.py: list_digests + save_patterns
- tests for signals, cluster, detect entrypoint

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_cluster.py

@@ -0,0 +1,54 @@
+"""Clusterer + evidence + cross-flavor tests (T05/T06)."""
+
+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 PROBLEM, SUCCESS, Signal  # noqa: E402
+
+
+def _sig(uid, flavor, repo, type_, polarity, locus, mag=1.0):
+    return Signal(session_uid=uid, flavor=flavor, repo=repo, type=type_,
+                  polarity=polarity, locus=locus, magnitude=mag)
+
+
+def test_min_frequency_filters_singletons():
+    sigs = [_sig("claude:a", "claude", "r1", "retry_storm", PROBLEM, "retries")]
+    assert cluster(sigs, min_frequency=2) == []
+
+
+def test_clusters_recurring_signal_with_evidence():
+    sigs = [
+        _sig("claude:a", "claude", "r1", "retry_storm", PROBLEM, "retries", 5),
+        _sig("claude:b", "claude", "r2", "retry_storm", PROBLEM, "retries", 3),
+    ]
+    pats = cluster(sigs, min_frequency=2)
+    assert len(pats) == 1
+    p = pats[0]
+    assert p.frequency == 2
+    assert p.sessions == ["claude:a", "claude:b"]
+    assert sorted(p.repos) == ["r1", "r2"]
+    assert p.flavors == ["claude"]
+    assert p.cross_flavor is False
+    assert p.cost_impact == 8.0
+
+
+def test_cross_flavor_flagged_and_ranked_first():
+    sigs = [
+        # cross-flavor problem (claude + codex)
+        _sig("claude:a", "claude", "r1", "repeated_errors", PROBLEM, "errors", 3),
+        _sig("codex:b", "codex", "r2", "repeated_errors", PROBLEM, "errors", 3),
+        # single-flavor success cluster with higher raw impact
+        _sig("grok:c", "grok", "r3", "clean_pass", SUCCESS, "outcome", 5),
+        _sig("grok:d", "grok", "r4", "clean_pass", SUCCESS, "outcome", 5),
+    ]
+    pats = cluster(sigs, min_frequency=2)
+    assert len(pats) == 2
+    xf = next(p for p in pats if p.signal_type == "repeated_errors")
+    assert xf.cross_flavor is True
+    assert sorted(xf.flavors) == ["claude", "codex"]
+    # cross-flavor pattern is ranked first even if another has higher raw impact
+    assert pats[0].cross_flavor is True
+    assert "cross-flavor" in pats[0].title