chore: remove unused imports and dead locals (ruff F401, F841) (#17010)

Mechanical cleanup across 43 files — removes 46 unused imports
(F401) and 14 unused local variables (F841) detected by
`ruff check --select F401,F841`. Net: -49 lines.

Also fixes a latent NameError in rl_cli.py where `get_hermes_home()`
was called at module line 32 before its import at line 65 — the
module never imported successfully on main. The ruff audit surfaced
this because it correctly saw the symbol as imported-but-unused
(the call happened before the import ran); the fix moves the import
to the top of the file alongside other stdlib imports.

One `# noqa: F401` kept in hermes_cli/status.py for `subprocess`:
tests monkeypatch `hermes_cli.status.subprocess` as a regression
guard that systemctl isn't called on Termux, so the name must
exist at module scope even though the module body doesn't reference
it. Docstring explains the reason.

Also fixes an invalid `# noqa:` directive in
gateway/platforms/discord.py:308 that lacked a rule code.

Co-authored-by: teknium1 <teknium@users.noreply.github.com>

gateway/platforms/feishu_comment.py

@@ -974,7 +974,6 @@ def build_whole_comment_prompt(
 
 def _resolve_model_and_runtime() -> Tuple[str, dict]:
     """Resolve model and provider credentials, same as gateway message handling."""
-    import os
     from gateway.run import _load_gateway_config, _resolve_gateway_model
 
     user_config = _load_gateway_config()