fix: use gitleaks CLI instead of licensed action, fix lint errors

- Replace gitleaks-action (requires paid license) with direct CLI
  invocation -- same engine, no license needed
- Fix ruff I001 import sorting in voice.py and test_llm.py
- Remove unused imports: _chat_request (test_llm), Path (test_music)
- Remove unused assignment: original_spawn (test_voice)

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

tests/test_voice.py

@@ -182,7 +182,6 @@ class TestSayCommand:
         msg = _Msg(text="!say hello world")
 
         spawned = []
-        original_spawn = bot._spawn
 
         def track_spawn(coro, *, name=None):
             spawned.append(name)