feat: add Tavily as opt-in search/extract provider

[tavily-integrations]

Summary

Adds Tavily as a configurable alternative to the default DuckDuckGo HTML scraper for web_search and web_extract tools. This is an additive change — DuckDuckGo remains the default; Tavily is purely opt-in via config.

What changed

• nipux_cli/web.py — Refactored web_search/web_extract into _ddg_search/_ddg_extract (DuckDuckGo) and added _tavily_search/_tavily_extract (Tavily SDK). Public functions now accept a search_provider parameter and dispatch accordingly.
• nipux_cli/config.py — Added search_provider: str field to ToolAccessConfig (default "duckduckgo"). load_config() reads tools.search_provider from YAML. Updated default_config_yaml() template.
• nipux_cli/tools.py — _web_call now reads ctx.config.tools.search_provider and passes it to the web dispatch layer (removed unused del ctx).
• pyproject.toml — Added tavily-python>=0.5.0 to dependencies.
• config.example.yaml — Documented search_provider option under tools.
• .env.example — Added TAVILY_API_KEY placeholder.

How to enable

1. Set TAVILY_API_KEY in ~/.nipux/.env
2. Add search_provider: tavily under tools: in config.yaml

Files changed

• nipux_cli/web.py
• nipux_cli/config.py
• nipux_cli/tools.py
• pyproject.toml
• config.example.yaml
• .env.example

Notes for reviewers

• Tavily client is lazily imported only when the provider is selected, so there is no startup cost when using DuckDuckGo.
• All existing DuckDuckGo behavior is preserved unchanged.

Automated Review

• Passed after 1 attempt(s)
• Final review: The migration cleanly adds Tavily as a configurable search provider alongside DuckDuckGo. The implementation is additive and correct: public functions web_search/web_extract are preserved with backward-compatible signatures, provider dispatch is clean, config threading through ToolContext is correct, TavilyClient() auto-reads TAVILY_API_KEY from the environment as expected, and existing tests remain valid since the new search_provider parameter defaults to "duckduckgo". All six reportedly changed files are accounted for in the diff. No regressions found.