feat: add Tavily web search as configurable alternative to Exa

[tavily-integrations]

Summary

• Added Tavily as a Tier-2 (API-key required) web search channel that runs in parallel with the existing free/zero-config Exa integration
• Tavily is additive — Exa remains untouched as the default search path
• The doctor command now surfaces both search channels independently

Files Changed

• agent_reach/channels/tavily_search.py (new) — TavilySearchChannel implementing the BaseChannel contract; validates tavily-python install and TAVILY_API_KEY presence
• agent_reach/channels/__init__.py — Import and register TavilySearchChannel in ALL_CHANNELS
• agent_reach/config.py — Add 'tavily_search': ['tavily_api_key'] to FEATURE_REQUIREMENTS
• agent_reach/channels/reddit.py — Expand backends list to include Exa and Tavily; update offline message to mention alternatives
• pyproject.toml — Add tavily-python>=0.3 as optional dep (new search extra, also added to all)
• agent_reach/guides/setup-tavily.md (new) — Setup guide for obtaining and configuring a Tavily API key

Dependency Changes

• Added tavily-python>=0.3 to [project.optional-dependencies] (search and all extras)

Environment Variable Changes

• Added TAVILY_API_KEY — read via Config.get('tavily_api_key') with uppercase env var fallback

Notes for Reviewers

• One pre-existing test failure in test_config.py::test_get_configured_features caused by GITHUB_TOKEN being set in CI environment — not related to this change
• All 77 other tests pass

🤖 Generated with Claude Code

Automated Review

• Passed after 1 attempt(s)
• Final review: The Tavily parallel channel implementation is correct, well-scoped, and consistent with codebase patterns. The new TavilySearchChannel follows the same structure as ExaSearchChannel (check-only, no search()/read() implementation, which is by design for this "glue layer" project). All required files are updated: channel module, registry, config feature requirements, pyproject.toml optional deps, and a setup guide. No regressions to existing functionality. Two minor issues found.