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)

tools/image_generation_tool.py

@@ -39,7 +39,7 @@ from urllib.parse import urlencode
 import fal_client
 from tools.debug_helpers import DebugSession
 from tools.managed_tool_gateway import resolve_managed_tool_gateway
-from tools.tool_backend_helpers import managed_nous_tools_enabled
+from tools.tool_backend_helpers import managed_nous_tools_enabled, prefers_gateway
 
 logger = logging.getLogger(__name__)
 
@@ -87,8 +87,9 @@ _managed_fal_client_lock = threading.Lock()
 
 
 def _resolve_managed_fal_gateway():
-    """Return managed fal-queue gateway config when direct FAL credentials are absent."""
-    if os.getenv("FAL_KEY"):
+    """Return managed fal-queue gateway config when the user prefers the gateway
+    or direct FAL credentials are absent."""
+    if os.getenv("FAL_KEY") and not prefers_gateway("image_gen"):
         return None
     return resolve_managed_tool_gateway("fal-queue")