feat: add Zhipu AI (GLM) model provider support

Adds zhipuai as a new provider using the OpenAI-compatible API at
api.z.ai. Supports api_key config or ZHIPUAI_API_KEY env var, with
optional endpoint override.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/config/schema.ts

@@ -19,7 +19,7 @@ const serverSchema = z.object({
 });
 
 const modelConfigBaseSchema = z.object({
-  provider: z.enum(['anthropic', 'openai', 'gemini', 'ollama', 'llamacpp', 'openrouter', 'bedrock', 'github']),
+  provider: z.enum(['anthropic', 'openai', 'gemini', 'ollama', 'llamacpp', 'openrouter', 'bedrock', 'github', 'zhipuai']),
   model: z.string(),
   endpoint: z.string().optional(),
   api_key: z.string().optional(),