feat(memory): implement memory indexing and retrieval engine (#94)

Add comprehensive indexing and retrieval system for memory search:
- VectorIndex for semantic similarity search using cosine similarity
- TemporalIndex for time-based queries with range and recency support
- EntityIndex for entity-based lookups with multi-entity intersection
- OutcomeIndex for success/failure filtering on episodes
- MemoryIndexer as unified interface for all index types
- RetrievalEngine with hybrid search combining all indices
- RelevanceScorer for multi-signal relevance scoring
- RetrievalCache for LRU caching of search results

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/memory/indexing/__init__.py

@@ -1,7 +1,56 @@
+# app/services/memory/indexing/__init__.py
 """
-Memory Indexing
+Memory Indexing & Retrieval.
 
-Vector embeddings and retrieval engine for memory search.
+Provides vector embeddings and multiple index types for efficient memory search:
+- Vector index for semantic similarity
+- Temporal index for time-based queries
+- Entity index for entity lookups
+- Outcome index for success/failure filtering
 """
 
-# Will be populated in #94
+from .index import (
+    EntityIndex,
+    EntityIndexEntry,
+    IndexEntry,
+    MemoryIndex,
+    MemoryIndexer,
+    OutcomeIndex,
+    OutcomeIndexEntry,
+    TemporalIndex,
+    TemporalIndexEntry,
+    VectorIndex,
+    VectorIndexEntry,
+    get_memory_indexer,
+)
+from .retrieval import (
+    CacheEntry,
+    RelevanceScorer,
+    RetrievalCache,
+    RetrievalEngine,
+    RetrievalQuery,
+    ScoredResult,
+    get_retrieval_engine,
+)
+
+__all__ = [
+    "CacheEntry",
+    "EntityIndex",
+    "EntityIndexEntry",
+    "IndexEntry",
+    "MemoryIndex",
+    "MemoryIndexer",
+    "OutcomeIndex",
+    "OutcomeIndexEntry",
+    "RelevanceScorer",
+    "RetrievalCache",
+    "RetrievalEngine",
+    "RetrievalQuery",
+    "ScoredResult",
+    "TemporalIndex",
+    "TemporalIndexEntry",
+    "VectorIndex",
+    "VectorIndexEntry",
+    "get_memory_indexer",
+    "get_retrieval_engine",
+]