chore: prune unused imports and duplicate import redefinitions

Remove unused imports (F401) and duplicate/shadowed import
redefinitions (F811) across the codebase using ruff's safe
autofixes. No behavioral changes -- imports only.

- ~1400 safe autofixes applied across 644 files (net -1072 lines)
- __init__.py re-exports preserved (excluded from F401 removal so
  public re-export surfaces stay intact)
- Re-exports that are imported or monkeypatched by tests but look
  unused in their defining module are kept with explicit # noqa:
  F401 (gateway/run.py load_dotenv; run_agent re-exports from
  agent.message_sanitization, agent.context_compressor,
  agent.retry_utils, agent.prompt_builder, agent.process_bootstrap,
  agent.codex_responses_adapter)
- Unsafe F841 (unused-variable) fixes deliberately skipped -- those
  can change behavior when the RHS has side effects
- ruff lints remain disabled in pyproject.toml (only PLW1514 is
  selected); this is a one-time cleanup, not a config change

Verification:
- python -m compileall: clean
- pytest --collect-only: all 27161 tests collect (zero import errors)
- core entry points import clean (run_agent, model_tools, cli,
  toolsets, hermes_state, batch_runner, gateway)
- static scan: every name any test imports directly from an edited
  module still resolves

tests/gateway/test_voice_command.py

@@ -905,7 +905,6 @@ class TestVoiceChannelCommands:
     @pytest.mark.asyncio
     async def test_input_no_adapter(self, runner):
         """No Discord adapter — early return, no crash."""
-        from gateway.config import Platform
         # No adapters set
         await runner._handle_voice_channel_input(111, 42, "Hello")
 
@@ -1355,7 +1354,7 @@ class TestCallbackWiringOrder:
 
     def test_callback_set_before_join(self):
         """_handle_voice_channel_join wires callback before calling join."""
-        import ast, inspect
+        import inspect
         from gateway.run import GatewayRunner
         source = inspect.getsource(GatewayRunner._handle_voice_channel_join)
         lines = source.split("\n")
@@ -1490,7 +1489,7 @@ class TestAutoTtsEmptyTextGuard:
 
     def test_base_empty_check_in_source(self):
         """base.py must check speech_text is non-empty before calling TTS."""
-        import ast, inspect
+        import inspect
         from gateway.platforms.base import BasePlatformAdapter
         source = inspect.getsource(BasePlatformAdapter._process_message_background)
         assert "if not speech_text" in source or "not speech_text" in source, (
@@ -2228,7 +2227,6 @@ class TestDisconnectVoiceCleanup:
 
     @pytest.mark.asyncio
     async def test_disconnect_clears_voice_state(self):
-        from unittest.mock import AsyncMock
 
         adapter = MagicMock()
         adapter._voice_clients = {111: MagicMock(), 222: MagicMock()}
@@ -2665,7 +2663,7 @@ class TestVoiceTTSPlayback:
 
     def _call_should_reply(self, runner, voice_mode, msg_type, response="Hello",
                            agent_msgs=None, already_sent=False):
-        from gateway.platforms.base import MessageType, MessageEvent, SessionSource
+        from gateway.platforms.base import MessageEvent, SessionSource
         from gateway.config import Platform
         runner._voice_mode["discord:ch1"] = voice_mode
         source = SessionSource(