feat: add Tavily as parallel search engine option

[tavily-integrations]

Summary

Adds Tavily as a third configurable --engine tavily search provider alongside the existing DuckDuckGo (default) and SearXNG options. This is an additive change — no existing providers are modified.

When --engine tavily is used, search results feed through the same StealthRetriever pipeline, so full-page stealth fetch, Evidence Records, and Bot Wall detection work identically to the SearXNG path.

Changes

pyproject.toml

• Added tavily-python>=0.5 under a new [tavily] optional extra

src/deepcloak/config.py

• Added 'tavily' to _VALID_ENGINE set
• Added tavily_api_key: str | None field to Settings dataclass
• resolve() now reads TAVILY_API_KEY from the environment

src/deepcloak/retriever.py

• Added tavily_search(api_key, query, max_results) using TavilyClient.search()
• Refactored build_stealth_retriever() to accept an optional search_fn callable (defaults to searxng_search for backward compatibility)

src/deepcloak/research_core.py

• Added a tavily branch in _run_ldr() that builds the StealthRetriever with tavily_search bound to the API key, gated on settings.tavily_api_key

src/deepcloak/cli.py

• Added 'tavily' to --engine choices

Dependency changes

• Added tavily-python>=0.5 as optional extra [tavily] in pyproject.toml

Environment variable changes

• Added TAVILY_API_KEY — read by config.resolve(); required when --engine tavily is used

Notes for reviewers

• The build_stealth_retriever() signature change is backward-compatible: searxng_url is now optional (with default None) and existing callers passing it as a keyword arg continue to work unchanged.
• tavily-python is opt-in via pip install deepcloak[tavily] — it is not added to core dependencies.

Automated Review

• Passed after 2 attempt(s)
• Final review: The Tavily migration unit is correct, complete, and consistent with the codebase. All 6 reported fixes are present: ConfigError on missing key, 'tavily'→'duckduckgo' in to_ldr_overrides(), pre-constructed TavilyClient passed via functools.partial, 3 config tests, 2 retriever tests, and README+pyproject.toml updates. The implementation follows the same patterns as the existing SearXNG integration. One minor concern: the broad except Exception: pass in research_core.py silently swallows ImportError if tavily-python is not installed, falling back to duckduckgo without user feedback — but this is consistent with the existing SearXNG branch and is not a blocker.

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