feat(coding-agent): add Qwen CLI OAuth provider

[4h9fbZ]

Adds a Qwen CLI provider extension with OAuth device flow support, matching the official Qwen CLI.

Changes:

• Add custom-provider-qwen-cli extension (packages/coding-agent/examples/extensions/custom-provider-qwen-cli)
• Add Qwen OAuth device flow with PKCE and refresh support
• Register Qwen models (coder plus/flash + vision) with OpenAI-compatible API
• Add thinkingFormat: "qwen" to core types (uses enable_thinking param)
• Enable modifyModels hook for extension OAuth providers (documented in packages/coding-agent/docs/custom-provider.md but not implemented; enables dynamic baseUrl from OAuth credentials)
• Register extension in example list + workspace

Models:

• qwen3-coder-plus: 1M context, 65K output
• qwen3-coder-flash: 1M context, 65K output
• vision-model (qwen3-vl-plus): 262K context, 32K output, vision + thinking

Testing:

• npm run check passes
• ./test.sh passes
• OAuth flow verified against official Qwen CLI source
• API calls and thinking mode tested with live tokens

[badlogic]

Merged to main. Thanks.