fix: remove unused imports and bare exception aliases (pyflakes F401/F841)

Remove unused imports across 86 files in src/, web_interface/, test/,
and scripts/ using autoflake. No logic changes — only dead import
statements and unused names in from-imports are removed.

Also remove bare exception aliases where the variable is never
referenced in the handler body:
- src/cache/disk_cache.py: except (IOError, OSError, PermissionError) as e
- src/cache_manager.py: except (OSError, IOError, PermissionError) as perm_error
- src/plugin_system/resource_monitor.py: except Exception as e
- web_interface/app.py: except Exception as read_err

86 files changed, 205 lines removed, 18 pre-existing test failures unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/test_plugin_system.py

@@ -1,12 +1,8 @@
-import pytest
-import os
-import sys
 import time
-from unittest.mock import MagicMock, patch, ANY, call
+from unittest.mock import MagicMock, patch
 from pathlib import Path
 from src.plugin_system.plugin_manager import PluginManager
 from src.plugin_system.plugin_state import PluginState
-from src.exceptions import PluginError
 
 class TestPluginManager:
     """Test PluginManager functionality."""
@@ -90,7 +86,6 @@ class TestPluginLoader:
         """Test dependency checking logic."""
         # This would test _check_dependencies_installed and _install_plugin_dependencies
         # which requires mocking subprocess calls and file operations
-        pass
 
 
 class TestPluginExecutor: