docs: add Manifest provider section

packages/web/src/content/docs/providers.mdx

@@ -1642,6 +1642,61 @@ OpenCode Zen is a list of tested and verified models provided by the OpenCode te
 
 ---
 
+### Manifest
+
+[Manifest](https://manifest.build) is an open-source LLM router that cuts inference costs through smart routing across 16+ providers. You get full control over which model handles each request. Route by complexity tier, task-specificity (coding, web browsing, etc.) and custom tiers.
+
+1. Head over to [manifest.build](https://manifest.build), create an account, and copy your API key (starts with `mnfst_`).
+
+2. Run the `/connect` command and search for Manifest.
+
+   ```txt
+   /connect
+   ```
+
+3. Enter the API key for the provider.
+
+   ```txt
+   ┌ API key
+   │
+   │
+   └ enter
+   ```
+
+4. Run the `/models` command and select `auto`.
+
+   ```txt
+   /models
+   ```
+
+5. You can also configure Manifest through your opencode config.
+
+   **Cloud** (default, no base URL needed):
+
+   ```json title="opencode.json"
+   {
+     "$schema": "https://opencode.ai/config.json",
+     "provider": {
+       "manifest": {}
+     }
+   }
+   ```
+
+   **Self-hosted** (Manifest is open-source, can run locally with Docker for fully private inference):
+
+   ```json title="opencode.json"
+   {
+     "$schema": "https://opencode.ai/config.json",
+     "provider": {
+       "manifest": {
+         "api": "http://localhost:2099/v1"
+       }
+     }
+   }
+   ```
+
+---
+
 ### LLM Gateway
 
 1. Head over to the [LLM Gateway dashboard](https://llmgateway.io/dashboard), click **Create API Key**, and copy the key.