Allow /insights to accept custom output path

Question

## 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/`