Add Kimi for Code provider support

[skeltavik]

Summary

Adds support for the Kimi for Code API endpoint (https://api.kimi.com/coding/v1), an Anthropic-compatible coding agent API with 262K context window.

Changes

• Add KIMI_CODING_BASE_URL constant in openfang-types
• Add kimi_coding provider using Anthropic driver (Anthropic-compatible endpoint)
• Add kimi-for-coding model entry (262K context, 32K output, subscription-based)
• Update provider count tests (36 → 37, 31 → 32)
• Add test for kimi_coding provider defaults

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes (for modified crates)
• cargo test --workspace passes (687 tests)
• Live integration tested - requires Kimi Code subscription

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries

API Key

Environment variable: KIMI_API_KEY

Usage

[default_model]
provider = "kimi_coding"
model = "kimi-for-coding"

Note

Kimi for Code uses a subscription-based "Coding Plan" (not per-token pricing) and is only available for authorized coding agents like Claude Code, Roo Code, etc.