docs: add Nous Tool Gateway documentation

- New page: user-guide/features/tool-gateway.md covering eligibility,
  setup (hermes model, hermes tools, manual config), how use_gateway
  works, precedence, switching back, status checking, self-hosted
  gateway env vars, and FAQ
- Added to sidebar under Features (top-level, before Core category)
- Cross-references from: overview.md, tools.md, browser.md,
  image-generation.md, tts.md, providers.md, environment-variables.md
- Added Nous Tool Gateway subsection to env vars reference with
  TOOL_GATEWAY_DOMAIN, TOOL_GATEWAY_SCHEME, TOOL_GATEWAY_USER_TOKEN,
  and FIRECRAWL_GATEWAY_URL

website/docs/reference/environment-variables.md

@@ -110,6 +110,17 @@ For native Anthropic auth, Hermes prefers Claude Code's own credential files whe
 | `WANDB_API_KEY` | RL training metrics ([wandb.ai](https://wandb.ai/)) |
 | `DAYTONA_API_KEY` | Daytona cloud sandboxes ([daytona.io](https://daytona.io/)) |
 
+### Nous Tool Gateway
+
+These variables configure the [Tool Gateway](/docs/user-guide/features/tool-gateway) for paid Nous subscribers or self-hosted gateway deployments. Most users don't need to set these — the gateway is configured automatically via `hermes model` or `hermes tools`.
+
+| Variable | Description |
+|----------|-------------|
+| `TOOL_GATEWAY_DOMAIN` | Base domain for Tool Gateway routing (default: `nousresearch.com`) |
+| `TOOL_GATEWAY_SCHEME` | HTTP or HTTPS scheme for gateway URLs (default: `https`) |
+| `TOOL_GATEWAY_USER_TOKEN` | Auth token for the Tool Gateway (normally auto-populated from Nous auth) |
+| `FIRECRAWL_GATEWAY_URL` | Override URL for the Firecrawl gateway endpoint specifically |
+
 ## Terminal Backend
 
 | Variable | Description |