feat: add browser tool (navigate, snapshot, click, back)

Zero-dep HTTP-based browser for web page interaction:
- browser_navigate(url) — fetch page, extract title/text/links/buttons
- browser_snapshot() — return current page view with ref IDs (@e1, @e2, ...)
- browser_click(ref) — follow a link by ref ID, resolves relative URLs
- browser_back() — navigate to previous page in history

HTML parsing via regex (stdlib only): extracts headings, paragraphs, list
items, links, buttons, and input elements. 10MB response cap, URL
validation, HTTP error handling, mutex-safe state for concurrent calls.

15 TDD tests + race detector clean.

Author: jkyberneees