feat: add Tavily as configurable search backend alongside TinyFish

[tavily-integrations]

Summary

Adds Tavily as an opt-in, configurable search backend alongside the existing TinyFish provider. When TAVILY_API_KEY is set (or SEARCH_PROVIDER=tavily), the service uses Tavily; otherwise it falls back to TinyFish. No existing code is removed.

Changes

• internal/search/tinyfish.go — Renamed Search() to SearchTinyFish() so the dispatcher can call providers by name.
• internal/search/tavily.go (new) — Implements SearchTavily() using Tavily's REST API (POST https://api.tavily.com/search), mapping response fields to the existing Result struct.
• internal/search/provider.go (new) — Top-level Search() dispatcher that reads SEARCH_PROVIDER env var (tavily|tinyfish) and auto-detects based on TAVILY_API_KEY presence when unset.
• cmd/wr/main.go — Added TAVILY_API_KEY and SEARCH_PROVIDER to cmdSetup() checks and usage() text.

Dependency Changes

• None — Tavily REST API is called with net/http (same pattern as TinyFish).

Environment Variable Changes

• TAVILY_API_KEY — Tavily API key (https://app.tavily.com)
• SEARCH_PROVIDER — Optional selector: tavily or tinyfish (auto-detected if omitted)

Notes for Reviewers

• internal/research/service.go calls search.Search() which now routes through the new dispatcher — no changes needed there.
• The MCP server (cmd/wr-mcp/main.go) inherits the new capability transitively.
• Go toolchain was not available in the build environment for compilation verification, but all references and types are consistent.

Automated Review

• Passed after 2 attempt(s)
• Final review: The migration correctly implements Tavily as an additive search provider alongside TinyFish. All four issues from the previous review cycle have been resolved: (1) cmdSetup() now requires at least one of TINYFISH_API_KEY or TAVILY_API_KEY, (2) the usage string is updated to "provider auto-detected", (3) http.StatusOK replaces the magic 200, and (4) the unused Score field is removed. The new provider.go dispatcher cleanly routes through the existing search.Search call surface, so both cmd/wr and cmd/wr-mcp (which go through research.Service) work without changes. No new external dependencies are needed since the Tavily client uses only stdlib. No regressions detected.

[mrvarmazyar]

@tavily-integrations thanks for contribution, please rebase the branch with main :)