feat(search): add SearXNG search provider support

[alanpeng]

Summary

Adds SearXNG as a third search provider for the built-in web_search tool (alongside Brave and Firecrawl). SearXNG is a self-hosted, privacy-respecting metasearch engine that exposes a JSON API — no API key required.

What changes

• Search settings (SearchSettings): new searxngBaseUrl field, default http://localhost:8080, persisted via the existing settings manager.
• Runtime (packages/runtime): web_search dispatches to the SearXNG JSON API (GET {base}/search?q=...&format=json&language=auto) with Accept: application/json and X-Forwarded-For/X-Real-IP: 127.0.0.1 headers. Non-OK responses surface the SearXNG error body instead of a bare status message.
• Settings UI (desktop): SearXNG option in the search provider dropdown + a service-URL field shown when SearXNG is selected.
• LangGraph generator parity: the generated Python web_search.py mirrors the TS implementation (same URL/params/headers/error handling), per the project's generator-parity requirement; generated sources regenerated.
• Codegen (generate-project-button): the generated project env now forwards SEARXNG_BASE_URL instead of writing undefined.
• Tests: TS + generated-Python coverage for query building, result parsing, non-OK error bodies, and settings persistence.

Configuration

Set the provider to SearXNG in Settings → Search and point the service URL at a self-hosted SearXNG instance (default http://localhost:8080), or set SEARXNG_BASE_URL in the environment.

[alanpeng]

Friendly ping for review — happy to iterate on anything.

Short version: adds searxng as a built-in web-search provider for self-hosted SearXNG instances — provider entry + base-URL field in search settings, JSON format API integration in the built-in web tools, and the matching backend in the LangGraph generator so generated projects stay in parity. No new npm dependencies; failures surface the service's error body instead of failing silently. Design notes and the test matrix are in the repo (docs/superpowers/specs/, packages/core/tests).

This is one of three open PRs from us (#150 Windows support, #152 i18n) — independent of each other, any review/merge order works.