Add BrowserTrace to observability tools

[aaronlab]

Adds BrowserTrace as a single observability-and-evaluation entry.

Why it fits this list:

• BrowserTrace is a local-first trace viewer for AI browser-agent failures.
• It records Browser Use, Stagehand, Skyvern, and Playwright + LLM runs with screenshots, model I/O, URLs, actions, errors, and public-safe HTML exports.
• It is MIT-licensed and installable from PyPI with pip install "browsertrace[ui]".

Validation:

• Edited entries.yaml only, per CONTRIBUTING.md.
• Parsed entries.yaml and categories.yaml with PyYAML; BrowserTrace appears once and observability-and-evaluation is a valid category.
• git diff --check passes.