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/normalize.py

@@ -27,7 +27,7 @@ def normalize(filepath: str) -> str:
     try:
         with open(filepath, "r", encoding="utf-8", errors="replace") as f:
             content = f.read()
-    except Exception as e:
+    except OSError as e:
         raise IOError(f"Could not read {filepath}: {e}")
 
     if not content.strip():
@@ -213,7 +213,7 @@ def _messages_to_transcript(messages: list, spellcheck: bool = True) -> str:
             from mempalace.spellcheck import spellcheck_user_text
 
             _fix = spellcheck_user_text
-        except Exception:
+        except ImportError:
             _fix = None
     else:
         _fix = None