feat: add Tavily as a parallel search engine option

[tavily-integrations]

Summary

• Added Tavily as an opt-in search engine alongside existing engines (Bing, DuckDuckGo, Brave, etc.)
• Tavily is selectable via DEFAULT_SEARCH_ENGINE=tavily or by including 'tavily' in the engines list
• Tavily engine is silently skipped if TAVILY_API_KEY is not set, preserving zero-config behavior

Files changed

• package.json — Added @tavily/core (^0.7.2) as a runtime dependency
• src/config.ts — Added 'tavily' to defaultSearchEngine type union and validSearchEngines array; added tavilyApiKey field reading TAVILY_API_KEY from environment
• src/core/search/searchEngines.ts — Added 'tavily' to SUPPORTED_SEARCH_ENGINES tuple and normalizeEngineName switch
• src/engines/tavily/tavily.ts — New file implementing SearchEngineExecutor using @tavily/core, mapping Tavily results to SearchResult[]
• src/runtime/createRuntime.ts — Registered Tavily executor in the engine map, conditional on TAVILY_API_KEY being set

Dependency changes

• Added @tavily/core ^0.7.2 to dependencies in package.json

Environment variable changes

• Added TAVILY_API_KEY (required to activate the Tavily engine)

Notes for reviewers

• All existing engines are untouched — this is a purely additive change
• Tavily API limits max results to 20, so the executor caps the limit accordingly
• TypeScript compiles cleanly with no errors

🤖 Generated with Claude Code

Automated Review

• Passed after 1 attempt(s)
• Final review: The Tavily engine integration is well-implemented and correct. The additive approach is consistent with the existing engine pattern. All required locations (config types, validation lists, searchEngines constants, normalizeEngineName switch, runtime registration) are updated. The @tavily/core SDK is used correctly, result mapping matches the SearchResult interface, and the conditional registration on TAVILY_API_KEY is the right pattern for optional API-key-gated engines. No regressions introduced.