fix: support Azure OpenAI deployment config

[euyua9]

Summary

• allow AZURE_OPENAI_DEPLOYMENT as a first-class config key instead of rejecting it as invalid
• route Azure OpenAI endpoints through deployment-based chat completion requests
• surface the deployment in config UI/model selection so Azure users can actually run the CLI

Testing

• npm install
• npm run typecheck
• npm run build
• HOME=$(mktemp -d) node dist/cli.mjs config set OPENAI_KEY=dummy AZURE_OPENAI_DEPLOYMENT=my-deploy
• HOME=$(mktemp -d) node dist/cli.mjs config get AZURE_OPENAI_DEPLOYMENT