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>

tests/test_chunker.py

@@ -0,0 +1,73 @@
+"""Tests for the markdown chunker."""
+
+from atocore.ingestion.chunker import chunk_markdown
+
+
+def test_basic_chunking():
+    """Test that markdown is split into chunks."""
+    body = """## Section One
+
+This is the first section with some content that is long enough to pass the minimum chunk size filter applied by the chunker.
+
+## Section Two
+
+This is the second section with different content that is also long enough to pass the minimum chunk size threshold.
+"""
+    chunks = chunk_markdown(body)
+    assert len(chunks) >= 2
+    assert all(c.char_count > 0 for c in chunks)
+    assert all(c.chunk_index >= 0 for c in chunks)
+
+
+def test_heading_path_preserved():
+    """Test that heading paths are captured."""
+    body = """## Architecture
+
+### Layers
+
+The system has multiple layers organized in a clear hierarchy for separation of concerns and maintainability.
+"""
+    chunks = chunk_markdown(body)
+    assert len(chunks) >= 1
+    # At least one chunk should have heading info
+    has_heading = any(c.heading_path for c in chunks)
+    assert has_heading
+
+
+def test_small_chunks_filtered():
+    """Test that very small chunks are discarded."""
+    body = """## A
+
+Hi
+
+## B
+
+This is a real section with enough content to pass the minimum size threshold.
+"""
+    chunks = chunk_markdown(body, min_size=50)
+    # "Hi" should be filtered out
+    for c in chunks:
+        assert c.char_count >= 50
+
+
+def test_large_section_split():
+    """Test that large sections are split further."""
+    large_content = "Word " * 200  # ~1000 chars
+    body = f"## Big Section\n\n{large_content}"
+    chunks = chunk_markdown(body, max_size=400)
+    assert len(chunks) >= 2
+
+
+def test_metadata_passed_through():
+    """Test that base metadata is included in chunks."""
+    body = "## Test\n\nSome content here that is long enough."
+    meta = {"source_file": "/test/file.md", "tags": ["test"]}
+    chunks = chunk_markdown(body, base_metadata=meta)
+    if chunks:
+        assert chunks[0].metadata.get("source_file") == "/test/file.md"
+
+
+def test_empty_body():
+    """Test chunking an empty body."""
+    chunks = chunk_markdown("")
+    assert chunks == []