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>

src/atocore/config.py

@@ -0,0 +1,39 @@
+"""AtoCore configuration via environment variables."""
+
+from pathlib import Path
+
+from pydantic_settings import BaseSettings
+
+
+class Settings(BaseSettings):
+    debug: bool = False
+    data_dir: Path = Path("./data")
+    host: str = "127.0.0.1"
+    port: int = 8100
+
+    # Embedding
+    embedding_model: str = (
+        "sentence-transformers/paraphrase-multilingual-MiniLM-L12-v2"
+    )
+
+    # Chunking
+    chunk_max_size: int = 800
+    chunk_overlap: int = 100
+    chunk_min_size: int = 50
+
+    # Context
+    context_budget: int = 3000
+    context_top_k: int = 15
+
+    model_config = {"env_prefix": "ATOCORE_"}
+
+    @property
+    def db_path(self) -> Path:
+        return self.data_dir / "atocore.db"
+
+    @property
+    def chroma_path(self) -> Path:
+        return self.data_dir / "chroma"
+
+
+settings = Settings()