Add web-search Pi extension (web_search + web_fetch tools)

Implements two LLM-callable tools using DuckDuckGo's free HTML
endpoint — no API key or account required:
- web_search: searches DDG, returns up to 8 results with title/URL/snippet
- web_fetch: fetches a URL, strips script/style/nav blocks, returns up
  to 12,000 chars of readable text

Both tools are TypeScript registered via pi.registerTool() and load
automatically from pi/agent/extensions/web-search/ via the ~/.pi symlink.
HTTP requests use Node.js built-ins (node:https) with a 15s timeout and
single-redirect follow.

Also adds an Extensions section to README.md listing all extensions
with a focused web-search subsection.

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