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/tools/test_send_message_tool.py

@@ -4,7 +4,6 @@ import asyncio
 import json
 import os
 import sys
-from pathlib import Path
 from types import SimpleNamespace
 from unittest.mock import AsyncMock, MagicMock, patch
 
@@ -40,7 +39,6 @@ from tools.send_message_tool import (
 # and provide a thin ``_send_discord(token, ...)`` shim that mirrors the
 # pre-migration signature so the existing test bodies keep working.
 from plugins.platforms.discord.adapter import (
-    _DISCORD_CHANNEL_TYPE_PROBE_CACHE,
     _derive_forum_thread_name,
     _probe_is_forum_cached,
     _remember_channel_is_forum,
@@ -1517,7 +1515,6 @@ class TestSendMatrixUrlEncoding:
 
     def test_room_id_is_percent_encoded_in_url(self):
         """Matrix room IDs with ! and : are percent-encoded in the PUT URL."""
-        import aiohttp
 
         mock_resp = MagicMock()
         mock_resp.status = 200
@@ -1894,10 +1891,6 @@ class TestForumProbeCache:
         discord_adapter._DISCORD_CHANNEL_TYPE_PROBE_CACHE.clear()
 
     def test_cache_round_trip(self):
-        from plugins.platforms.discord.adapter import (
-            _probe_is_forum_cached,
-            _remember_channel_is_forum,
-        )
         assert _probe_is_forum_cached("xyz") is None
         _remember_channel_is_forum("xyz", True)
         assert _probe_is_forum_cached("xyz") is True