/insights: time-based comparisons so you can see what's actually improving

Currently /insights runs over what appears to be a fixed time window (roughly 30 days) with no way to adjust the range or compare periods.

Request

• Display the analysis time period explicitly in the report
• Allow a date range parameter (e.g. /insights 7d, /insights 30d)
• Side-by-side or delta comparison between periods (e.g. "last 7d vs. previous 7d") to track whether workflow changes are actually improving friction patterns

Why it matters

The current snapshot format makes it hard to tell if improvements (like adding gotchas to CLAUDE.md) are actually reducing friction, or if they introduced new issues. Shorter intervals would create a feedback loop — make a change, see the result.