Allow /insights to accept custom output path

Feature Request

The /insights command currently saves the report to a fixed location:
~/.claude/usage-data/report.html

Requested behavior:
Allow specifying a custom output path, e.g.:

/insights ~/Desktop/insights.html
/insights --output ~/Desktop/insights.html

Why:
Users often want the report somewhere accessible (like Desktop) without needing to copy it manually after generation.

Current workaround:
cp ~/.claude/usage-data/report.html ~/Desktop/