[Feature Request] Add date range filtering and incremental analysis to /insights command

Feature Request: Date range filtering for /insights

I run /insights weekly, but every run re-analyzes the full 30-day
window back to the same start date. There's no way to see just what's
changed since the last run.

What I'd like:

• /insights --since 2026-03-09 or similar date range flag
• Ideally, automatic incremental mode that remembers the last run date

and only analyzes new sessions

• Bonus: a diff/comparison view showing how patterns changed

week-over-week

Why it matters:

• Weekly runs produce nearly identical reports with marginal new data

buried in the same old analysis

• Makes it hard to track whether friction points are improving or

getting worse over time

• Power users who run this regularly get diminishing value from

repeated full-window analysis