feat: Add support for Kimi For Coding model

[clawaizhang]

📝 Description

Add Claude Code User-Agent headers to bypass Coding Agent restriction for Kimi For Coding model. When using kimi-for-coding model or accessing kimi.com/coding endpoint, the provider now sends User-Agent: claude-cli/2.1.39 and x-app: cli headers to identify as a Coding Agent.

🗣️ Type of Change

• ✨ New feature (non-breaking change which adds functionality)

🤖 AI Code Generation

• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)

🔗 Related Issue

Fixes #1293

📚 Technical Context (Skip for Docs)

• Reference URL: https://www.kimi.com/coding/docs/more/third-party-agents.html
• Reasoning: Kimi For Coding API restricts access to specific Coding Agents (Claude Code, Roo Code, Kilo Code, etc.). By adding the required User-Agent and x-app headers, PicoClaw can now use Kimi For Coding model through the OpenAI-compatible endpoint.

🧪 Test Environment

• Hardware: PC
• OS: Windows 11
• Model/Provider: Kimi For Coding (api.kimi.com/coding)
• Channels: N/A

📸 Evidence (Optional)

Tested with configuration:

{
  "model_name": "kimi-coding",
  "model": "kimi-for-coding",
  "api_base": "https://api.kimi.com/coding/v1",
  "api_key": "sk-xxx"
}
```json

☑️ Checklist
 My code/docs follow the style of this project.
 I have performed a self-review of my own changes.
 I have updated the documentation accordingly.

[clawaizhang]

👤 Reviewers

/cc @yinwm 麻烦帮忙 review 一下这个 PR，是关于添加 Kimi For Coding 支持的 Provider 修改。谢谢！

[CLAassistant]

All committers have signed the CLA.

[yinwm]

Code Review Summary

Thanks for the contribution!

Issues

• Major: Protocol routing issue - kimi-for-coding without prefix falls into default case. Fix: Add special handling in openai case before merging.
• Medium: Hardcoded version 2.1.39 - Extract as constant
• Medium: Imprecise string matching - Use more precise matching
• Medium: Missing unit tests

Suggestion

Create kimi.go in anthropic_messages directory to centralize Kimi logic.

[yinwm]

Code Review Summary

Thanks for the contribution!

Issues

Severity	Issue	Suggestion
🔴 Major	Protocol routing: kimi-for-coding without prefix falls into default case	Add special handling at the beginning of openai case
🟡 Medium	Hardcoded version 2.1.39 will become outdated	Extract as a constant
🟡 Medium	Contains may match unintended URLs	Use more precise matching
🟡 Medium	Missing unit tests	Add test coverage

Suggestion

Create kimi.go in anthropic_messages directory:

pkg/providers/anthropic_messages/
├── provider.go
├── kimi.go          # Kimi-specific logic
└── provider_test.go

This keeps Kimi logic centralized, easier to maintain and test.

[clawaizhang]

@yinwm thanks for your comments, I add kimi providers to keeps Kimi logic centralized, easier to maintain and test.