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/entity_detector.py

@@ -660,7 +660,7 @@ def detect_entities(file_paths: list, max_files: int = 10) -> dict:
             all_text.append(content)
             all_lines.extend(content.splitlines())
             files_read += 1
-        except Exception:
+        except OSError:
             continue
 
     combined_text = "\n".join(all_text)