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/test_model_tools_async_bridge.py

@@ -14,7 +14,7 @@ import asyncio
 import json
 import threading
 from types import SimpleNamespace
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import AsyncMock, patch
 
 import pytest
 
@@ -129,7 +129,6 @@ class TestRunAsyncWorkerThread:
     def test_parallel_workers_get_separate_loops(self):
         """Different worker threads must get their own loops to avoid
         contention (the original reason for the worker-thread branch)."""
-        import time
         from concurrent.futures import ThreadPoolExecutor, as_completed
         from model_tools import _run_async
 
@@ -356,7 +355,7 @@ class TestVisionDispatchLoopSafety:
     def test_vision_dispatch_keeps_loop_alive(self, tmp_path):
         """After dispatching vision_analyze via the registry, the event
         loop must remain open so cached async clients don't crash on GC."""
-        from model_tools import _run_async, _get_tool_loop
+        from model_tools import _get_tool_loop
         from tools.registry import registry
 
         fake_response = _mock_vision_response()