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/conftest.py

@@ -0,0 +1,114 @@
+"""pytest configuration and shared fixtures."""
+
+import os
+import tempfile
+from pathlib import Path
+
+import pytest
+
+# Force test data directory
+os.environ["ATOCORE_DATA_DIR"] = tempfile.mkdtemp(prefix="atocore_test_")
+os.environ["ATOCORE_DEBUG"] = "true"
+
+
+@pytest.fixture
+def tmp_data_dir(tmp_path):
+    """Provide a temporary data directory for tests."""
+    os.environ["ATOCORE_DATA_DIR"] = str(tmp_path)
+    # Reset singletons
+    from atocore import config
+    config.settings = config.Settings()
+
+    import atocore.retrieval.vector_store as vs
+    vs._store = None
+
+    return tmp_path
+
+
+@pytest.fixture
+def sample_markdown(tmp_path) -> Path:
+    """Create a sample markdown file for testing."""
+    md_file = tmp_path / "test_note.md"
+    md_file.write_text(
+        """---
+tags:
+  - atocore
+  - architecture
+date: 2026-04-05
+---
+# AtoCore Architecture
+
+## Overview
+
+AtoCore is a personal context engine that enriches LLM interactions
+with durable memory, structured context, and project knowledge.
+
+## Layers
+
+The system has these layers:
+
+1. Main PKM (human, messy, exploratory)
+2. AtoVault (system mirror)
+3. AtoDrive (trusted project truth)
+4. Structured Memory (DB)
+5. Semantic Retrieval (vector DB)
+
+## Memory Types
+
+AtoCore supports these memory types:
+
+- Identity
+- Preferences
+- Project Memory
+- Episodic Memory
+- Knowledge Objects
+- Adaptation Memory
+- Trusted Project State
+
+## Trust Precedence
+
+When sources conflict:
+
+1. Trusted Project State wins
+2. AtoDrive overrides PKM
+3. Most recent confirmed wins
+4. Higher confidence wins
+5. Equal → flag conflict
+
+No silent merging.
+""",
+        encoding="utf-8",
+    )
+    return md_file
+
+
+@pytest.fixture
+def sample_folder(tmp_path, sample_markdown) -> Path:
+    """Create a folder with multiple markdown files."""
+    # Already has test_note.md from sample_markdown
+    second = tmp_path / "second_note.md"
+    second.write_text(
+        """---
+tags:
+  - chunking
+---
+# Chunking Strategy
+
+## Approach
+
+Heading-aware recursive splitting:
+
+1. Split on H2 boundaries first
+2. If section > 800 chars, split on H3
+3. If still > 800 chars, split on paragraphs
+4. Hard split at 800 chars with 100 char overlap
+
+## Parameters
+
+- max_chunk_size: 800 characters
+- overlap: 100 characters
+- min_chunk_size: 50 characters
+""",
+        encoding="utf-8",
+    )
+    return tmp_path