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/observability/logger.py

@@ -0,0 +1,41 @@
+"""Structured logging for AtoCore."""
+
+import logging
+
+import structlog
+
+from atocore.config import settings
+
+_LOG_LEVELS = {
+    "DEBUG": logging.DEBUG,
+    "INFO": logging.INFO,
+    "WARNING": logging.WARNING,
+    "ERROR": logging.ERROR,
+}
+
+
+def setup_logging() -> None:
+    """Configure structlog with JSON output."""
+    log_level = "DEBUG" if settings.debug else "INFO"
+
+    structlog.configure(
+        processors=[
+            structlog.contextvars.merge_contextvars,
+            structlog.processors.add_log_level,
+            structlog.processors.TimeStamper(fmt="iso"),
+            structlog.dev.ConsoleRenderer()
+            if settings.debug
+            else structlog.processors.JSONRenderer(),
+        ],
+        wrapper_class=structlog.make_filtering_bound_logger(
+            _LOG_LEVELS.get(log_level, logging.INFO)
+        ),
+        context_class=dict,
+        logger_factory=structlog.PrintLoggerFactory(),
+        cache_logger_on_first_use=True,
+    )
+
+
+def get_logger(name: str) -> structlog.BoundLogger:
+    """Get a named logger."""
+    return structlog.get_logger(name)