feat(core): Engram, Store, Retriever, CLI - Grundsystem Second Brain

- src/engram.py: Gedaechtniseinheit mit Confidence, Correctness, Links
- src/store.py: SQLite FTS5 persistenter Speicher
- src/retriever.py: Hybrid Suche + Reranking
- src/cli.py: Kommandozeilen-Interface

Issue: #1

src/retriever.py

@@ -0,0 +1,55 @@
+"""
+Hybrid-Retrieval Engine.
+Phase 1: FTS-Keyword + Confidence-Reranking.
+Phase 2: + Embedding + Fusion.
+"""
+
+from typing import List, Dict, Any
+from .engram import Engram
+from .store import EngramStore
+
+
+class Retriever:
+    def __init__(self, store: EngramStore):
+        self.store = store
+
+    def retrieve(
+        self,
+        query: str,
+        limit: int = 5,
+        min_confidence: float = 0.0,
+        source_filter: str = None,
+        tag_filter: str = None,
+    ) -> List[Dict[str, Any]]:
+        results = []
+        keyword_results = self.store.search_text(query, limit=limit * 3)
+        for eg in keyword_results:
+            conf = eg.compute_confidence()
+            if conf < min_confidence:
+                continue
+            if source_filter and eg.metadata.get("source") != source_filter:
+                continue
+            if tag_filter and tag_filter not in eg.metadata.get("tags", []):
+                continue
+            eg.touch()
+            self.store.save(eg)
+            results.append({"engram": eg, "score": conf, "match_type": "keyword"})
+        results.sort(key=lambda r: r["score"], reverse=True)
+        return results[:limit]
+
+    def related(self, engram_id: str, limit: int = 5) -> List[Engram]:
+        eg = self.store.get(engram_id)
+        if not eg:
+            return []
+        out = []
+        for lid in eg.links:
+            linked = self.store.get(str(lid))
+            if linked:
+                out.append(linked)
+        return sorted(out, key=lambda e: e.compute_confidence(), reverse=True)[:limit]
+
+    def recent(self, limit: int = 10) -> List[Engram]:
+        return self.store.get_all(limit=limit)
+
+    def stats(self) -> Dict[str, Any]:
+        return self.store.stats()