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/retrieval/retriever.py

@@ -0,0 +1,83 @@
+"""Retrieval: query → ranked chunks."""
+
+import time
+from dataclasses import dataclass
+
+from atocore.config import settings
+from atocore.observability.logger import get_logger
+from atocore.retrieval.embeddings import embed_query
+from atocore.retrieval.vector_store import get_vector_store
+
+log = get_logger("retriever")
+
+
+@dataclass
+class ChunkResult:
+    chunk_id: str
+    content: str
+    score: float
+    heading_path: str
+    source_file: str
+    tags: str
+    title: str
+    document_id: str
+
+
+def retrieve(
+    query: str,
+    top_k: int | None = None,
+    filter_tags: list[str] | None = None,
+) -> list[ChunkResult]:
+    """Retrieve the most relevant chunks for a query."""
+    top_k = top_k or settings.context_top_k
+    start = time.time()
+
+    query_embedding = embed_query(query)
+    store = get_vector_store()
+
+    # Build filter
+    where = None
+    if filter_tags:
+        # ChromaDB where filter for tags (stored as JSON string)
+        # Simple contains check — works for single-tag filtering
+        where = {"tags": {"$contains": filter_tags[0]}}
+
+    results = store.query(
+        query_embedding=query_embedding,
+        top_k=top_k,
+        where=where,
+    )
+
+    chunks = []
+    if results and results["ids"] and results["ids"][0]:
+        for i, chunk_id in enumerate(results["ids"][0]):
+            # ChromaDB returns distances (lower = more similar for cosine)
+            # Convert to similarity score (1 - distance)
+            distance = results["distances"][0][i] if results["distances"] else 0
+            score = 1.0 - distance
+            meta = results["metadatas"][0][i] if results["metadatas"] else {}
+            content = results["documents"][0][i] if results["documents"] else ""
+
+            chunks.append(
+                ChunkResult(
+                    chunk_id=chunk_id,
+                    content=content,
+                    score=round(score, 4),
+                    heading_path=meta.get("heading_path", ""),
+                    source_file=meta.get("source_file", ""),
+                    tags=meta.get("tags", "[]"),
+                    title=meta.get("title", ""),
+                    document_id=meta.get("document_id", ""),
+                )
+            )
+
+    duration_ms = int((time.time() - start) * 1000)
+    log.info(
+        "retrieval_done",
+        query=query[:100],
+        top_k=top_k,
+        results_count=len(chunks),
+        duration_ms=duration_ms,
+    )
+
+    return chunks