feat: implement AtoCore Phase 0 + Phase 0.5 (foundation + PoC)

Complete implementation of the personal context engine foundation:
- FastAPI server with 5 endpoints (ingest, query, context/build, health, debug)
- SQLite database with 5 tables (documents, chunks, memories, projects, interactions)
- Heading-aware markdown chunker (800 char max, recursive splitting)
- Multilingual embeddings via sentence-transformers (EN/FR)
- ChromaDB vector store with cosine similarity retrieval
- Context builder with project boosting, dedup, and budget enforcement
- CLI scripts for batch ingestion and test prompt evaluation
- 19 unit tests passing, 79% coverage
- Validated on 482 real project files (8383 chunks, 0 errors)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.env.example

@@ -0,0 +1,8 @@
+ATOCORE_DEBUG=false
+ATOCORE_DATA_DIR=./data
+ATOCORE_HOST=127.0.0.1
+ATOCORE_PORT=8100
+ATOCORE_EMBEDDING_MODEL=sentence-transformers/paraphrase-multilingual-MiniLM-L12-v2
+ATOCORE_CHUNK_MAX_SIZE=800
+ATOCORE_CHUNK_OVERLAP=100
+ATOCORE_CONTEXT_BUDGET=3000