Add websearch provider selection to settings UI / config instead of only env var

Feature Request

The websearch tool currently supports two hosted providers: Exa (mcp.exa.ai) and Parallel (search.parallel.ai). The provider can only be pinned via the OPENCODE_WEBSEARCH_PROVIDER=exa|parallel environment variable; otherwise it is chosen randomly per session (via a session-ID checksum).

Problem

1. There is no option in the desktop GUI settings (Settings > Appearance has themes/fonts/shell/keybinds, no websearch section).
2. There is no field in opencode.json either.
3. Because of the random default, users can get noticeably worse results: in my testing, Exa consistently returned fresher and cleaner results (same-day news, official sources, compact highlights), while Parallel returned stale results and noisy raw-page dumps.
4. On Windows, setting a user env var also requires a full sign-out/restart to take effect for GUI-launched apps, which is not a great onboarding experience for a simple preference.

Suggestion

Add a first-class way to select the websearch provider, for example:

• A dropdown in the desktop settings UI (e.g. Settings > Tools / Web Search: auto (random) / exa / parallel), and/or
• A config option in opencode.json, e.g.:

{
  "websearch": { "provider": "exa" }
}

The config-based approach would also help users pin the provider across machines. A "auto" default would preserve current behavior.

Environment

• opencode desktop 1.18.10 (Windows 11, Electron build)
• Config layer currently only reads OPENCODE_WEBSEARCH_PROVIDER / EXA_API_KEY / PARALLEL_API_KEY from process.env (defaultConfigLayer in the websearch tool module)