feat: add Tavily as a configurable search provider

[tavily-integrations]

Summary

• Added Tavily as a fourth search provider option alongside DuckDuckGo (default), Brave, and SearXNG
• Tavily is opt-in via --search-provider tavily or DEEPWORM_SEARCH_PROVIDER=tavily
• Uses search_depth="advanced" and include_raw_content=True for research-quality results
• Existing providers remain fully intact and unchanged

Files changed

• pyproject.toml — Added tavily-python>=0.3.0 to core dependencies
• deepworm/search.py — Added _search_tavily() function and wired it into search_web() dispatch
• deepworm/config.py — Added "tavily" to VALID_SEARCH_PROVIDERS set
• deepworm/__main__.py — Added "tavily" to --search-provider CLI choices

Dependency changes

• Added tavily-python>=0.3.0 to pyproject.toml dependencies

Environment variable changes

• TAVILY_API_KEY — Required when search_provider=tavily (validated at search time, same pattern as BRAVE_API_KEY)

Notes for reviewers

• The _search_tavily() function returns results in the same SearchResult schema as other providers, so researcher.py requires no changes
• API key validation happens at search time in _search_tavily(), consistent with how _search_brave() validates BRAVE_API_KEY
• Falls back to DuckDuckGo if Tavily search fails, matching existing fallback behavior

🤖 Generated with Claude Code

Automated Review

• Passed after 1 attempt(s)
• Final review: The Tavily provider migration is a clean, additive implementation that correctly integrates Tavily as a fourth search provider. All four files from the plan are modified, SDK usage is correct, and existing providers are completely untouched. There are two minor issues: tavily-python is added as a mandatory core dependency rather than an optional extra (unlike anthropic and google which are optional), and the body field is not serialized in the search cache — though the latter is a pre-existing limitation affecting all providers.