fix: narrow bare except Exception to specific types where safe

Replace broad except Exception with specific exception types in 6
sites where the expected failure mode is well-defined:

- normalize.py: OSError for file read, ImportError for optional import
- miner.py: OSError for file read_text
- entity_detector.py: OSError for file read in scan loop
- convo_miner.py: (OSError, ValueError) for normalize which reads
  and parses files
- entity_registry.py: (URLError, OSError, JSONDecodeError, KeyError)
  for Wikipedia lookup fallback

ChromaDB except Exception sites (~30) are left broad for now.
chromadb.errors defines NotFoundError, DuplicateIDError,
InvalidDimensionException etc., but narrowing those sites requires
importing from chromadb.errors and validating across supported
versions (>=0.4.0). MCP server handlers also left broad for
resilience.

mempalace/convo_miner.py

@@ -301,7 +301,7 @@ def mine_convos(
         # Normalize format
         try:
             content = normalize(str(filepath))
-        except Exception:
+        except (OSError, ValueError):
             continue
 
         if not content or len(content.strip()) < MIN_CHUNK_SIZE: