feat: add Tavily as configurable search provider alongside DuckDuckGo

[tavily-integrations]

Summary

Adds Tavily as an optional, configurable web search strategy alongside the existing DuckDuckGo+Puppeteer implementation. When TAVILY_API_KEY is set as an environment secret, the research workflow automatically uses Tavily's search API (with advanced search depth and raw markdown content) instead of browser-based DuckDuckGo scraping. When no key is configured, the existing DuckDuckGo+Puppeteer path is used unchanged.

Changes

New functionality

• src/webSearch.ts: Added tavilySearch() function using @tavily/core SDK that returns results matching the existing SearchResult interface
• src/workflows.ts: Added createTavilySearchStrategy() factory and wired it into ResearchWorkflow.run() — selects Tavily when TAVILY_API_KEY is present
• src/config.ts: Added tavilySearch config section with maxResults and searchDepth settings
• src/bindings.d.ts: Added optional TAVILY_API_KEY to the Env type
• src/logger.ts: Extended logger source type to include "tavily"
• .dev.vars.example: Added TAVILY_API_KEY placeholder

Files changed

• src/webSearch.ts
• src/workflows.ts
• src/bindings.d.ts
• src/config.ts
• src/logger.ts
• .dev.vars.example
• package.json
• package-lock.json

Dependency changes

• Added @tavily/core (^0.7.3) — official Tavily JS/TS SDK

Environment variable changes

• Added TAVILY_API_KEY (optional secret) — when absent, falls back to DuckDuckGo+Puppeteer

Notes for reviewers

• The existing DuckDuckGo+Puppeteer code and BROWSER binding are fully preserved
• The SearchStrategy interface already supported multiple providers; Tavily slots in cleanly
• TypeScript compiles cleanly and Biome lint passes
• Tavily results include rawContent as markdown, eliminating the need for Puppeteer-based content extraction

Automated Review

• Passed after 1 attempt(s)
• Final review: The Tavily migration is well-structured and functionally correct. The additive approach preserves the existing DuckDuckGo+Puppeteer path as a fallback, environment variable gating works correctly, and all Tavily SDK usage patterns are valid. The nodejs_compat flag is already present in wrangler.jsonc, so @tavily/core's axios dependency will work in Cloudflare Workers. The logger type unions, bindings declaration, and config additions are all consistent. Three minor issues were found but none block approval.