feat: add kimi-web-search builtin extension

[ckanner]

Summary

Closes #9

This PR adds a new builtin extension kimi-web-search that integrates Kimi Code platform's official SearchWeb and FetchURL services into senpi.

Problem

• senpi currently has no web search integration for the kimi-coding provider
• The existing anthropic-web-search extension incorrectly injects Anthropic-native web_search_20250305 into kimi-coding requests (both use api: "anthropic-messages"), which Kimi API does not support
• Kimi Code platform provides official moonshot_search/moonshot_fetch tools that should be utilized

Solution

New Files

• packages/coding-agent/src/core/extensions/builtin/kimi-web-search/index.ts (278 lines)
  • Registers SearchWeb and FetchURL tools via pi.registerTool()
  • Calls Kimi Code service endpoints with proper headers (Authorization, X-Msh-Tool-Call-Id)
  • Falls back to local HTTP fetch on error
  • Configurable via PI_KIMI_WEB_SEARCH, PI_KIMI_SEARCH_BASE_URL, PI_KIMI_FETCH_BASE_URL

Modified Files

• packages/coding-agent/src/core/extensions/builtin/index.ts
  • Registers kimi-web-search as builtin extension
• packages/coding-agent/src/core/extensions/changes.md
  • Documents the fork change and expected merge conflict zones

Testing

• Verified SearchWeb returns search results with text_query, limit, enable_page_crawling
• Verified FetchURL fetches web pages with fallback to local HTTP
• Tested with PI_ANTHROPIC_WEB_SEARCH=0 to ensure no conflict with anthropic-web-search

Checklist

• Extension follows existing builtin extension patterns
• Fork change documented in changes.md
• Pre-commit checks pass (Biome + tsgo + browser-smoke + web-ui)
• Atomic commits: feature code separate from documentation
• No generated files or lockfile changes included

---

Summary by cubic

Add a built-in kimi-web-search extension integrating Kimi Code’s SearchWeb and FetchURL for the kimi-coding provider, enabling web search and URL fetch via Kimi’s official APIs. Closes #9.

• New Features

  • Registers SearchWeb and FetchURL tools calling https://api.kimi.com/coding/v1/search and /fetch with Authorization and X-Msh-Tool-Call-Id (includes timeout_seconds: 30).
  • Config via PI_KIMI_WEB_SEARCH, PI_KIMI_SEARCH_BASE_URL, PI_KIMI_FETCH_BASE_URL; falls back to local HTTP fetch on service errors.
  • Resolves API key with ctx.modelRegistry.getApiKeyAndHeaders(); exposes results/content to the agent.
  • Registered in the builtin registry as id kimi-web-search.

• Bug Fixes

  • Stops injecting Anthropic web_search_20250305 into kimi-coding requests; uses Kimi services instead.

^{Written for commit 9ce5adc. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

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