feat(web-search): add optional youcom search provider

[mouse-value-add]

What

Adds You.com as an opt-in web_search provider, following the exact same pattern as the existing brave / tavily / kagi / olostep providers in WebSearchTool.

Why

Memmy's agent runtime leans on web_search for anything current — docs lookups, research tasks, fresh context for the memory-driven workflows. The built-in DuckDuckGo path works but is HTML-scraped and rate-limit-prone (the code already defends against bot challenges), and the current keyed providers don't include a citation-first web search option. You.com's Search API returns ranked web results with snippets via a plain JSON endpoint, which slots cleanly into the existing provider switch.

Changes

• src/core/agent-runtime/tools/web.ts: new youcom provider — POST https://ydc-index.io/v1/search with the X-API-Key header, results mapped through the existing formatResults helper. Key resolution follows the other providers: tools.web.search.apiKey first, then YDC_API_KEY. Without a key it falls back to DuckDuckGo, so default behavior is completely unchanged.
• src/entrypoints/frontend-bridge/settings-api.ts: adds You.com to WEB_SEARCH_PROVIDER_OPTIONS so it shows up in the desktop settings provider list (credential: "api_key").
• tests/core/agent-runtime/tools/web-search-tool.test.ts: 5 new cases mirroring the Brave/Tavily tests — request shape (endpoint, X-API-Key, body), snippet-over-description preference, description fallback, error reporting, and the no-key → DuckDuckGo fallback.
• docs/en/tools/mcp.mdx + docs/cn/tools/mcp.mdx: documents the full provider list and where keys come from (the provider list wasn't written down anywhere before).

Setup

tools:
  web:
    enabled: true
    search:
      provider: youcom
      apiKey: ${YDC_API_KEY}
      maxResults: 5

API keys are available at you.com/platform/api-keys. I kept this entirely opt-in — no new dependencies, no config migration, nothing changes unless you explicitly set provider: youcom.

Validation

• npx vitest run on the touched suites: 112 tests passing (30 in web-search-tool.test.ts, 10 in settings-api.test.ts, plus tool-loader / searchusage / build-status).
• npm run typecheck clean.

Notes

For anyone who'd rather use You.com through MCP than the REST API, You.com also exposes a hosted MCP server (https://api.you.com/mcp) that works with Memmy's existing tools.mcpServers streamableHttp config — the provider here is just the lighter path when you want results inline in web_search.

Happy to adjust the shape if you'd prefer a different provider name or want the key env var renamed.