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_signals.py

@@ -0,0 +1,53 @@
+"""Signal extractor tests (T04)."""
+
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+from session_memory.detect.signals import (  # noqa: E402
+    PROBLEM, SUCCESS, build_context, extract_signals,
+)
+
+
+def _digest(uid, flavor="claude", repo="r", outcome="success", tokens=100,
+            errors=0, retries=0, test_runs=0):
+    return {
+        "session_uid": uid, "flavor": flavor, "repo": repo, "outcome": outcome,
+        "cost": {"input_tokens": tokens, "output_tokens": 0},
+        "markers": {"errors": errors, "retries": retries, "test_runs": test_runs,
+                    "edits": 0, "human_interventions": 0},
+    }
+
+
+def test_problem_signals():
+    digests = [
+        _digest("claude:a", retries=5, outcome="fail"),
+        _digest("claude:b", errors=4),
+        _digest("claude:c", outcome="abandoned"),
+    ]
+    sigs = extract_signals(digests)
+    types = {(s.session_uid, s.type) for s in sigs}
+    assert ("claude:a", "retry_storm") in types
+    assert ("claude:b", "repeated_errors") in types
+    assert ("claude:c", "abandoned") in types
+    assert all(s.polarity == PROBLEM for s in sigs
+               if s.type in ("retry_storm", "repeated_errors", "abandoned"))
+
+
+def test_success_signals():
+    sigs = extract_signals([_digest("grok:x", outcome="success", test_runs=2)])
+    assert any(s.type == "clean_pass" and s.polarity == SUCCESS for s in sigs)
+
+    rec = extract_signals([_digest("codex:y", outcome="success", errors=2)])
+    assert any(s.type == "error_then_recovery" and s.polarity == SUCCESS for s in rec)
+
+
+def test_budget_overrun_uses_corpus_p90():
+    digests = [_digest(f"claude:{i}", tokens=100) for i in range(10)]
+    digests.append(_digest("claude:big", tokens=100000))
+    ctx = build_context(digests)
+    assert ctx["tokens_p90"] >= 100
+    sigs = extract_signals(digests, ctx)
+    overruns = [s for s in sigs if s.type == "budget_overrun"]
+    assert overruns and overruns[0].session_uid == "claude:big"