fix: reject partial browse client env integers

[hiSandog]

Summary

• require GSTACK_PORT and BROWSE_TAB env values to be full integer strings before accepting them
• keep the bundled hackernews browser-skill SDK copy in sync with the canonical browse-client
• add regression coverage for trailing-character env values

Tests

• /Users/sandog/.real/.bin/bun test browse/test/browse-client.test.ts
• git diff --check

Note: the first sandboxed test run failed before assertions because Bun.serve(port: 0) could not listen inside the sandbox; the same command passed outside the sandbox.

---

^{Need help on this PR? Tag @codesmith with what you need.}

• Let Codesmith autofix CI failures and bot reviews