docs: Add Z.AI Coding Plan provider example

## Summary
- Add zai-coding to Providers table
- Add Z.AI Coding Plan to All Supported Vendors table
- Add Z.AI Coding Plan configuration example with troubleshooting note

docs/providers.md

@@ -11,6 +11,7 @@
 | ------------ | --------------------------------------- | ------------------------------------------------------------ |
 | `gemini`     | LLM (Gemini direct)                     | [aistudio.google.com](https://aistudio.google.com)           |
 | `zhipu`      | LLM (Zhipu direct)                      | [bigmodel.cn](https://bigmodel.cn)                           |
+| `zai-coding` | LLM (Z.AI Coding Plan)                | [docs.z.ai](https://docs.z.ai/devpack/overview)           |
 | `volcengine` | LLM(Volcengine direct)                  | [volcengine.com](https://www.volcengine.com/activity/codingplan?utm_campaign=PicoClaw&utm_content=PicoClaw&utm_medium=devrel&utm_source=OWO&utm_term=PicoClaw)                 |
 | `openrouter` | LLM (recommended, access to all models) | [openrouter.ai](https://openrouter.ai)                       |
 | `anthropic`  | LLM (Claude direct)                     | [console.anthropic.com](https://console.anthropic.com)       |
@@ -47,6 +48,7 @@ This design also enables **multi-agent support** with flexible provider selectio
 | **OpenAI**          | `openai/`         | `https://api.openai.com/v1`                         | OpenAI    | [Get Key](https://platform.openai.com)                           |
 | **Anthropic**       | `anthropic/`      | `https://api.anthropic.com/v1`                      | Anthropic | [Get Key](https://console.anthropic.com)                         |
 | **智谱 AI (GLM)**   | `zhipu/`          | `https://open.bigmodel.cn/api/paas/v4`              | OpenAI    | [Get Key](https://open.bigmodel.cn/usercenter/proj-mgmt/apikeys) |
+| **Z.AI Coding Plan** | `openai/`         | `https://api.z.ai/api/coding/paas/v4`              | OpenAI    | [Get Zhipu Key](https://bigmodel.cn/usercenter/proj-mgmt/apikeys) |
 | **DeepSeek**        | `deepseek/`       | `https://api.deepseek.com/v1`                       | OpenAI    | [Get Key](https://platform.deepseek.com)                         |
 | **Google Gemini**   | `gemini/`         | `https://generativelanguage.googleapis.com/v1beta`  | OpenAI    | [Get Key](https://aistudio.google.com/api-keys)                  |
 | **Groq**            | `groq/`           | `https://api.groq.com/openai/v1`                    | OpenAI    | [Get Key](https://console.groq.com)                              |
@@ -238,6 +240,21 @@ For direct Anthropic API access or custom endpoints that only support Anthropic'
 
 PicoClaw strips only the outer `litellm/` prefix before sending the request, so proxy aliases like `litellm/lite-gpt4` send `lite-gpt4`, while `litellm/openai/gpt-4o` sends `openai/gpt-4o`.
 
+**Z.AI Coding Plan**
+
+If the standard Zhipu endpoint (`https://open.bigmodel.cn/api/paas/v4`) returns 429 (code 1113: insufficient balance), try using the Z.AI Coding Plan endpoint instead:
+
+```json
+{
+  "model_name": "glm-4.7",
+  "model": "openai/glm-4.7",
+  "api_key": "your-zhipu-api-key",
+  "api_base": "https://api.z.ai/api/coding/paas/v4"
+}
+```
+
+**Note:** The Z.AI Coding Plan endpoint and standard Zhipu endpoint use the same API key format but have separate billing. If you encounter 429 errors with the standard Zhipu endpoint, the Z.AI Coding Plan endpoint may have available balance.
+
 #### Load Balancing
 
 Configure multiple endpoints for the same model name—PicoClaw will automatically round-robin between them: