feat: HTML side-by-side diff viewer for forge compare

[saggda]

Problem

forge compare outputs text. Hard to read two diffs side by side in the terminal for large changes.

Proposed solution

Add --html flag that generates a static HTML file with side-by-side diff view:

forge compare 42 --html          # opens in browser
forge compare 42 --html --out report.html

Notes

• No external dependencies — use Python's difflib.HtmlDiff
• Self-contained single HTML file
• Good first issue — isolated, stdlib only