fix: prevent agent/bot review comments from overflowing their containers

[harivansh-afk]

Summary

Fixes #116

Long lines in bot/agent review comments (file paths, URLs, inline code) were breaking out of the comment bubble and annotation wrapper divs, causing the garbled rendering shown in the issue screenshot.

• Add break-words to the Markdown component so all long text wraps within its container
• Add overflow-hidden to the review comment thread wrapper in the PR activity timeline (pull-detail-activity.tsx)
• Add overflow-hidden to the review comment thread wrapper in the file diff review view (review-file-diff-block.tsx)

Test plan

• Open a PR that has review comments from a bot/agent (e.g. Devin, Copilot, CodeRabbit)
• Verify review comments with long lines (file paths, code suggestions) no longer overflow outside their container
• Verify normal human review comments still render correctly
• Check both the PR activity timeline and the file diff review view