Add --days / --since flag to /insights to scope analysis window

Feature Request

Summary

The /insights command currently analyzes a fixed ~30-day window of session data. There is no way to scope it to a shorter period (e.g. the past 7 days), making it less useful for users who run it frequently — the report surfaces the same patterns week after week rather than fresh observations.

Proposed API

/insights --days 7
/insights --since 2026-04-05

Both flags would filter the session data used for analysis to the specified window before generating the report. The HTML output filename could reflect the window (e.g. report-7d.html).

Motivation

I run /insights weekly and archive reports to a personal knowledge base. After the first few runs, the 30-day rolling window means each new report is ~80% overlap with the previous one — the same friction patterns, the same suggestions, the same interaction style summary. A --days 7 option would make each weekly report genuinely incremental.

Alternatives Considered

• Manually moving session files out of ~/.claude/usage-data/ before running — works but is fragile and not user-friendly
• Running insights less frequently — defeats the purpose of weekly reflection

Environment

• Claude Code version: latest
• OS: Arch Linux