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/terminal_tool.py

@@ -762,8 +762,8 @@ def _create_environment(env_type: str, image: str, cwd: str, timeout: int,
             if modal_state["managed_mode_blocked"]:
                 raise ValueError(
                     "Modal backend is configured for managed mode, but "
-                    "HERMES_ENABLE_NOUS_MANAGED_TOOLS is not enabled and no direct "
-                    "Modal credentials/config were found. Enable the feature flag or "
+                    "a paid Nous subscription is required for the Tool Gateway and no direct "
+                    "Modal credentials/config were found. Log in with `hermes model` or "
                     "choose TERMINAL_MODAL_MODE=direct/auto."
                 )
             if modal_state["mode"] == "managed":
@@ -1577,8 +1577,8 @@ def check_terminal_requirements() -> bool:
                 if modal_state["managed_mode_blocked"]:
                     logger.error(
                         "Modal backend selected with TERMINAL_MODAL_MODE=managed, but "
-                        "HERMES_ENABLE_NOUS_MANAGED_TOOLS is not enabled and no direct "
-                        "Modal credentials/config were found. Enable the feature flag "
+                        "a paid Nous subscription is required for the Tool Gateway and no direct "
+                        "Modal credentials/config were found. Log in with `hermes model` "
                         "or choose TERMINAL_MODAL_MODE=direct/auto."
                     )
                     return False