Add persistent memory: Step 1-3 implementation

- Step 1: persist_signature() - append-only JSONL persistence
- Step 2: encode_to_phase() - sentence transformer phase encoding
         (fallback to zeros if model unavailable)
- Step 3: retrieve_signatures(), compute_resonance_score(), phase_distance()

- Added origin/parent_id fields to TemporalSignature schema
- Added sentence-transformers to requirements.txt

requirements.txt

@@ -5,6 +5,9 @@
 numpy>=1.24.0
 scipy>=1.10.0
 
+# Sentence transformers for phase encoding
+sentence-transformers>=2.2.0
+
 # Async
 asyncio>=3.4.3