Feature request: date range filter for /insights

Summary

The /insights command currently analyses all available sessions with no way to scope the analysis to a specific time period. A date range filter would allow users to focus on recent usage patterns and track improvement over time.

Proposed solution

Add --since and/or --until flags to /insights:

/insights --since 30d        # last 30 days
/insights --since 2026-04-01 # since a specific date
/insights --since 7d --until 2026-05-01  # specific window

Why this is useful

• Track improvement over time — after changing CLAUDE.md rules or adding hooks, you want to see whether friction events decreased in just the recent sessions, not diluted across months of historical data
• Compare periods — run insights for last month vs. this month to see trends
• Reduce noise — older sessions may reflect a different codebase, team, or workflow that's no longer relevant

Current workaround

None — the only option would be manually deleting older session files from ~/.claude/usage-data/facets/, which is destructive.