chore: remove unused imports and dead locals (ruff F401/F841 sweep)

Cleans F401 unused imports and F841 dead local assignments across
root *.py, agent/, hermes_cli/, tools/, gateway/, cron/, tui_gateway/
(tests/, plugins/, skills/ excluded).

Intentionally KEPT (false positives / test-patch surfaces):
- agent/transports/__init__.py package re-exports
- cli.py browser_connect re-exports (DEFAULT_BROWSER_CDP_URL area,
  used by tests/cli/test_cli_browser_connect.py)
- hermes_cli/main.py _prompt_auth_credentials_choice /
  _model_flow_bedrock_api_key (accessed via main_mod attr in tests)
- gateway/run.py aliased replay_cleanup + whatsapp_identity re-exports
  and _PORT_BINDING_PLATFORM_VALUES (test-referenced)
- hermes_cli/web_server.py get_running_pid (tests monkeypatch it) and
  _OAUTH_TOKEN_URL availability probe
- hermes_cli/config.py get_process_hermes_home re-export (noqa'd F811
  chain) and yaml availability-probe import
- hermes_cli/nous_subscription.py managed_nous_tools_enabled
  (tests patch hermes_cli.nous_subscription.managed_nous_tools_enabled)
- try/except ImportError availability probes (env_loader, tts_tool,
  mcp_tool, web_server anthropic OAuth block)
- tools/web_tools.py noqa F401 re-exports
- hermes_cli/setup_whatsapp_cloud.py:263 'proceed' skipped: possible
  missing-guard bug, flagged for separate review
- unused function parameters (signature changes out of scope)

Side-effect RHS calls preserved where only the binding was dead
(e.g. web_server proc = _spawn_hermes_action -> bare call).

agent/learning_graph_render.py

@@ -403,7 +403,6 @@ def _category_counts(payload: dict[str, Any]) -> list[tuple[str, int]]:
 def category_color_map(payload: dict[str, Any]) -> dict[str, str]:
     """Deterministic, evenly-spread hue per skill category (theme-independent)."""
     clusters = _category_counts(payload)
-    n = max(1, len(clusters))
     # Golden-angle hue spacing so adjacent categories never collide in color.
     return {cat: rgb_to_hex(_hsl_to_rgb((i * 137.508) % 360, 0.55, 0.62)) for i, (cat, _c) in enumerate(clusters)}