feat: add xai realtime transcription

docs/plugins/voice-call.md

@@ -155,8 +155,8 @@ Current runtime behavior:
 
 - `streaming.provider` is optional. If unset, Voice Call uses the first
   registered realtime transcription provider.
-- Today the bundled provider is OpenAI, registered by the bundled `openai`
-  plugin.
+- Bundled realtime transcription providers include OpenAI (`openai`) and xAI
+  (`xai`), registered by their provider plugins.
 - Provider-owned raw config lives under `streaming.providers.<providerId>`.
 - If `streaming.provider` points at an unregistered provider, or no realtime
   transcription provider is registered at all, Voice Call logs a warning and
@@ -169,6 +169,15 @@ OpenAI streaming transcription defaults:
 - `silenceDurationMs`: `800`
 - `vadThreshold`: `0.5`
 
+xAI streaming transcription defaults:
+
+- API key: `streaming.providers.xai.apiKey` or `XAI_API_KEY`
+- endpoint: `wss://api.x.ai/v1/stt`
+- `encoding`: `mulaw`
+- `sampleRate`: `8000`
+- `endpointingMs`: `800`
+- `interimResults`: `true`
+
 Example:
 
 ```json5
@@ -197,6 +206,33 @@ Example:
 }
 ```
 
+Use xAI instead:
+
+```json5
+{
+  plugins: {
+    entries: {
+      "voice-call": {
+        config: {
+          streaming: {
+            enabled: true,
+            provider: "xai",
+            streamPath: "/voice/stream",
+            providers: {
+              xai: {
+                apiKey: "${XAI_API_KEY}", // optional if XAI_API_KEY is set
+                endpointingMs: 800,
+                language: "en",
+              },
+            },
+          },
+        },
+      },
+    },
+  },
+}
+```
+
 Legacy keys are still auto-migrated by `openclaw doctor --fix`:
 
 - `streaming.sttProvider` → `streaming.provider`