Add date-range filtering to /insights

Feature Request

Add date-range or timeframe filtering to the /insights command so users can analyze specific windows of their usage history.

Why This Matters

The current /insights command analyzes all available sessions (~30 days) as a single block. But usage patterns, friction points, and workflows evolve over time. Being able to compare insights across different timeframes would surface habit changes — for example:

• "Am I hitting fewer hallucination issues this week vs. last month?"
• "Did adding that custom skill actually reduce my correction cycles?"
• "How has my workflow shifted since I started using MCP servers?"

Without timeframe filtering, there's no way to track whether workflow improvements are actually working or whether friction patterns are trending up or down.

Suggested API

# Filter by number of days
/insights --days 15

# Filter by date range
/insights --since 2026-02-13
/insights --from 2026-02-01 --to 2026-02-15

# Compare two periods
/insights --compare 15d   # compares last 15 days vs. prior 15 days

Bonus: Period Comparison

The most powerful version would support a --compare flag that runs insights on two adjacent time windows and highlights what changed — new friction patterns, resolved issues, shifts in project focus, etc. This would make /insights a true feedback loop for improving how users work with Claude Code.