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>

session_memory/core/store.py

@@ -223,6 +223,22 @@ class Store:
         row = self.db.execute("SELECT json FROM digests WHERE session_uid=?", (session_uid,)).fetchone()
         return json.loads(row["json"]) if row else None
 
+    def list_digests(self) -> list[dict[str, Any]]:
+        return [json.loads(r["json"]) for r in self.db.execute("SELECT json FROM digests")]
+
+    def save_patterns(self, patterns: list[dict[str, Any]]) -> None:
+        """Persist candidate patterns to a Tier 2 table (replace prior run)."""
+        self.db.execute(
+            "CREATE TABLE IF NOT EXISTS patterns ("
+            "key TEXT PRIMARY KEY, json TEXT NOT NULL, detected_at TEXT NOT NULL)"
+        )
+        self.db.execute("DELETE FROM patterns")
+        self.db.executemany(
+            "INSERT INTO patterns(key, json, detected_at) VALUES(?,?,?)",
+            [(p["key"], json.dumps(p, sort_keys=True), _now()) for p in patterns],
+        )
+        self.db.commit()
+
     # ---- reads -------------------------------------------------------------
 
     def get_session(self, session_uid: str) -> Optional[Session]: