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_slash_access_dispatch.py

@@ -337,7 +337,6 @@ async def test_running_agent_fastpath_blocks_non_admin_command():
     )
     src = _make_source(user_id="999")
     # Mark the session as having an in-flight agent so the fast-path runs.
-    from gateway.session import build_session_key
     sk = build_session_key(src)
     runner._running_agents[sk] = MagicMock()
     runner._running_agents_ts[sk] = 0  # not stale (epoch + small delta on this machine)
@@ -361,7 +360,6 @@ async def test_running_agent_fastpath_allows_admin_command():
         }
     )
     src = _make_source(user_id="111")  # admin
-    from gateway.session import build_session_key
     sk = build_session_key(src)
     runner._running_agents[sk] = MagicMock()
     runner._running_agents_ts[sk] = 0
@@ -384,7 +382,6 @@ async def test_running_agent_fastpath_status_always_works():
         }
     )
     src = _make_source(user_id="999")  # non-admin
-    from gateway.session import build_session_key
     sk = build_session_key(src)
     runner._running_agents[sk] = MagicMock()
     runner._running_agents_ts[sk] = 0