refactor(src): narrow exception types in Phase 6 batch (8 sites across 3 files)

Migrates the 8 try/except sites in provider + adapter + orchestration
files by narrowing the exception types from broad 'except Exception' to
specific stdlib/domain exceptions.

Files and sites:
1. src/aggregate.py:50 (is_absolute_with_drive - PureWindowsPath)
   except Exception -> except (ValueError, OSError)
2. src/aggregate.py:105 (stats - ast.parse for element count)
   except Exception -> except (SyntaxError, ValueError)
3. src/aggregate.py:107 (stats outer try)
   except Exception -> except (OSError, SyntaxError)
4. src/aggregate.py:274 (file read with traceback)
   except Exception -> except (OSError, UnicodeDecodeError)
5. src/aggregate.py:446 (AST skeleton fallback)
   except Exception -> except (AttributeError, TypeError, ValueError)
6. src/multi_agent_conductor.py:317 (persona load fallback)
   except: -> except (OSError, KeyError, AttributeError, TypeError)
7. src/multi_agent_conductor.py:467 (persona apply with print)
   except Exception -> except (OSError, KeyError, AttributeError, TypeError)
8. src/multi_agent_conductor.py:517 (file view injection)
   except Exception -> except (OSError, UnicodeDecodeError, AttributeError, TypeError)
9. src/multi_agent_conductor.py:635 (response push with traceback)
   except Exception -> except (OSError, TypeError, AttributeError)
10. src/models.py:1081 (MCP config load)
    except Exception -> except (OSError, json.JSONDecodeError, UnicodeDecodeError)

Decisions (documented as no-op):
- src/dag_engine.py: 0 violations; 1 compliant site; no migration
- src/models.py:268 (RAISE AttributeError): legitimate __getattr__ pattern; keep
- src/gemini_cli_adapter.py:173-174 (RAISE): try/except + raise for SDK
  exception conversion; keep as-is (cascading if changed)
- src/conductor_tech_lead.py:120 UNCLEAR (Phase 2 decision): keep
- src/openai_compatible.py:87 UNCLEAR (Phase 2 decision): keep

Tests verified:
- tests/test_aggregate_flags.py (2 tests) PASS
- tests/test_context_composition_phase6.py (9 tests) PASS
- tests/test_tiered_context.py (5 tests) PASS
- tests/test_ui_summary_only_removal.py (6 tests) PASS
- tests/test_orchestration_logic.py (8 tests) PASS
- tests/test_dag_engine.py (9 tests) PASS
- tests/test_conductor_tech_lead.py (9 tests) PASS

src/models.py

@@ -1078,7 +1078,7 @@ def load_mcp_config(path: str) -> MCPConfiguration:
   try:
    data = json.load(f)
    return MCPConfiguration.from_dict(data)
-  except Exception:
+  except (OSError, json.JSONDecodeError, UnicodeDecodeError):
    return MCPConfiguration()
 
 #endregion: MCP Config