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_context_builder.py

@@ -0,0 +1,60 @@
+"""Tests for the context builder."""
+
+from atocore.context.builder import build_context, get_last_context_pack
+from atocore.ingestion.pipeline import ingest_file
+from atocore.models.database import init_db
+
+
+def test_build_context_returns_pack(tmp_data_dir, sample_markdown):
+    """Test that context builder returns a valid pack."""
+    init_db()
+    ingest_file(sample_markdown)
+
+    pack = build_context("What is AtoCore?")
+    assert pack.total_chars > 0
+    assert len(pack.chunks_used) > 0
+    assert pack.budget_remaining >= 0
+    assert "--- AtoCore Context ---" in pack.formatted_context
+    assert "--- End Context ---" in pack.formatted_context
+
+
+def test_context_respects_budget(tmp_data_dir, sample_markdown):
+    """Test that context builder respects character budget."""
+    init_db()
+    ingest_file(sample_markdown)
+
+    pack = build_context("What is AtoCore?", budget=500)
+    assert pack.total_chars <= 500
+
+
+def test_context_with_project_hint(tmp_data_dir, sample_markdown):
+    """Test that project hint boosts relevant chunks."""
+    init_db()
+    ingest_file(sample_markdown)
+
+    pack = build_context("What is the architecture?", project_hint="atocore")
+    assert len(pack.chunks_used) > 0
+    # With project hint, we should still get results
+    assert pack.total_chars > 0
+
+
+def test_last_context_pack_stored(tmp_data_dir, sample_markdown):
+    """Test that last context pack is stored for debug."""
+    init_db()
+    ingest_file(sample_markdown)
+
+    build_context("test prompt")
+    last = get_last_context_pack()
+    assert last is not None
+    assert last.query == "test prompt"
+
+
+def test_full_prompt_structure(tmp_data_dir, sample_markdown):
+    """Test that the full prompt has correct structure."""
+    init_db()
+    ingest_file(sample_markdown)
+
+    pack = build_context("What are memory types?")
+    assert "knowledge base" in pack.full_prompt.lower()
+    assert "--- AtoCore Context ---" in pack.full_prompt
+    assert "What are memory types?" in pack.full_prompt