feat(web-search): add Tavily provider

[savvadesogle]

Summary

Adds Tavily as a first-class os.web.search provider alongside the existing duckduckgo, searxng, exa and brave providers.

Motivation

os.web.search was unreliable on proxied / cloud-egress setups:

1. Exa without an API key. When EXA_API_KEY is unset, Exa falls back to its keyless MCP tier (https://mcp.exa.ai/mcp), which returns 502 / 403 / 429 under load. The project already tracks this degradation mode in Keyless Exa search returns HTTP 429 continuously and degrades to a weaker provider silently #179 (a GAIA campaign logged 1341 Exa returned HTTP 429 failures).
2. DuckDuckGo anti-bot wall. The keyless fallback (html.duckduckgo.com) intermittently serves an anti-bot challenge (HTTP 202 + challenge-form / anomaly-modal). This happens consistently from datacenter / VPN / proxy egress IPs, so the fallback is not a reliable backstop in proxied environments (verified 5/5 attempts, both direct and through a proxy).
3. Tavily is a stable, developer-friendly search API that requires an API key but behaves predictably and returns structured results[].

Changes

File	Change
src/tools/os/web-search/providers/tavily-provider.ts	New. createTavilyProvider() POSTs to https://api.tavily.com/search with { api_key, query, max_results }, throws on HTTP >= 400, and maps the results[] payload (title, url, content, publishedDate) to WebSearchResult[]. parseTavilyJson is exported for tests.
src/tools/os/web-search/web-search-provider.ts	Add "tavily" to the WebSearchProviderName union.
src/tools/os/web-search/providers/provider-registry.ts	Register createTavilyProvider under the "tavily" case in resolveProviderByName.
src/tools/os/web-search/providers/search-orchestrator.ts	isProviderUsable("tavily") requires TAVILY_API_KEY to be present in the environment (mirrors the brave gate), so a keyless Tavily is skipped rather than failing every search.
src/config/config-schema.ts	Add web.search.tavily.apiKeyEnv to WebSearchConfig, its default "TAVILY_API_KEY" in USER_CONFIG_DEFAULTS, parsing of the provider name union, and the webSearchTavily extraction in parseUserConfigFile.
src/tools/os/web-search/tool/warn-missing-search-key.ts	Treat Tavily as a keyed provider (add to KEYED_PROVIDERS and resolve its apiKeyEnv), so a missing key produces the same one-line startup warning as Exa/Brave.

Usage

// <stateDir>/config.json
"web": {
  "search": {
    "provider": "tavily",
    "fallback": []
  }
}

# <stateDir>/.env
TAVILY_API_KEY=tvly-...

Verification

• npm run lint — clean.
• npm test — full suite: 20 failed / 5939 passed. This is identical to the pre-existing baseline on main (all 20 failures are environment-specific Windows tests: clear-stale-chrome-locks, fs-approval-scope, open-terminal-window, send-message-concurrency, logo-art.generated, bootstrap, stream-cli-completion, skill-script-runner, skill-tools). No new failures are introduced.
• Manual: os.web.search returns Tavily results through a proxy.

Notes

• Tavily requests go through searchHttp, which shells out to curl. curl reads HTTP_PROXY / HTTPS_PROXY / NO_PROXY from the environment itself, so Tavily works behind a proxy without any extra wiring.
• Follows the exact shape of the existing brave / searxng providers, so the orchestrator, cache, retry policy, and error surface behave the same.