feat: add z.ai Coding Plan as built-in model provider

[iamlordutkarsh]

Add GLM-5.2, GLM-5.1, and GLM-4.6 models accessible via z.ai Coding Plan endpoint (https://api.z.ai/api/coding/paas/v4). Models are activated by setting ZAI_API_KEY environment variable.

• Add ZAIURL constant and ZAI_API_KEY env var in llm/oai/oai.go
• Add three GLM-zai Model definitions and registry entries
• Add catalog entries in models/models.go with z.ai base URL
• Add ZAIEnv() source in modelsources/modelsources.go to ensure z.ai models use ZAI_API_KEY instead of OPENAI_API_KEY
• Wire up ZAI_API_KEY in cmd/shelley/main.go across all source paths

[cla-bot]

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[philz]

I'm moving away from using environment variables to control models. We've got the add/remove models dialogs which can support z.ai as custom models. I'm open to improvements to that dialog box to make it easier, but I consider the env variable approach deprecated at the moment. Thanks.