fix: critical bugs and hardening from validation audit

- Fix infinite loop in chunker _hard_split when overlap >= max_size
- Fix tag filter false positives by quoting tag values in ChromaDB query
- Fix score boost semantics (additive → multiplicative) to stay within 0-1 range
- Add error handling and type hints to all API routes
- Update README with proper project documentation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/atocore/ingestion/chunker.py

@@ -137,6 +137,10 @@ def _split_by_paragraphs(
 
 def _hard_split(text: str, max_size: int, overlap: int) -> list[str]:
     """Hard split text at max_size with overlap."""
+    # Prevent infinite loop: overlap must be less than max_size
+    if overlap >= max_size:
+        overlap = max_size // 4
+
     chunks = []
     start = 0
     while start < len(text):