feat: ungate Tool Gateway — subscription-based access with per-tool opt-in

Replace the HERMES_ENABLE_NOUS_MANAGED_TOOLS env-var feature flag with
subscription-based detection. The Tool Gateway is now available to any
paid Nous subscriber without needing a hidden env var.

Core changes:
- managed_nous_tools_enabled() checks get_nous_auth_status() +
  check_nous_free_tier() instead of an env var
- New use_gateway config flag per tool section (web, tts, browser,
  image_gen) records explicit user opt-in and overrides direct API
  keys at runtime
- New prefers_gateway(section) shared helper in tool_backend_helpers.py
  used by all 4 tool runtimes (web, tts, image gen, browser)

UX flow:
- hermes model: after Nous login/model selection, shows a curses
  prompt listing all gateway-eligible tools with current status.
  User chooses to enable all, enable only unconfigured tools, or skip.
  Defaults to Enable for new users, Skip when direct keys exist.
- hermes tools: provider selection now manages use_gateway flag —
  selecting Nous Subscription sets it, selecting any other provider
  clears it
- hermes status: renamed section to Nous Tool Gateway, added
  free-tier upgrade nudge for logged-in free users
- curses_radiolist: new description parameter for multi-line context
  that survives the screen clear

Runtime behavior:
- Each tool runtime (web_tools, tts_tool, image_generation_tool,
  browser_use) checks prefers_gateway() before falling back to
  direct env-var credentials
- get_nous_subscription_features() respects use_gateway flags,
  suppressing direct credential detection when the user opted in

Removed:
- HERMES_ENABLE_NOUS_MANAGED_TOOLS env var and all references
- apply_nous_provider_defaults() silent TTS auto-set
- get_nous_subscription_explainer_lines() static text
- Override env var warnings (use_gateway handles this properly now)

tests/agent/test_prompt_builder.py

@@ -413,7 +413,7 @@ class TestBuildSkillsSystemPrompt:
 
 class TestBuildNousSubscriptionPrompt:
     def test_includes_active_subscription_features(self, monkeypatch):
-        monkeypatch.setenv("HERMES_ENABLE_NOUS_MANAGED_TOOLS", "1")
+        monkeypatch.setattr("tools.tool_backend_helpers.managed_nous_tools_enabled", lambda: True)
         monkeypatch.setattr(
             "hermes_cli.nous_subscription.get_nous_subscription_features",
             lambda config=None: NousSubscriptionFeatures(
@@ -437,7 +437,7 @@ class TestBuildNousSubscriptionPrompt:
         assert "do not ask the user for Firecrawl, FAL, OpenAI TTS, or Browser-Use API keys" in prompt
 
     def test_non_subscriber_prompt_includes_relevant_upgrade_guidance(self, monkeypatch):
-        monkeypatch.setenv("HERMES_ENABLE_NOUS_MANAGED_TOOLS", "1")
+        monkeypatch.setattr("tools.tool_backend_helpers.managed_nous_tools_enabled", lambda: True)
         monkeypatch.setattr(
             "hermes_cli.nous_subscription.get_nous_subscription_features",
             lambda config=None: NousSubscriptionFeatures(
@@ -460,7 +460,7 @@ class TestBuildNousSubscriptionPrompt:
         assert "Do not mention subscription unless" in prompt
 
     def test_feature_flag_off_returns_empty_prompt(self, monkeypatch):
-        monkeypatch.delenv("HERMES_ENABLE_NOUS_MANAGED_TOOLS", raising=False)
+        monkeypatch.setattr("tools.tool_backend_helpers.managed_nous_tools_enabled", lambda: False)
 
         prompt = build_nous_subscription_prompt({"web_search"})