Feature request: Add --since/--until options to /insights command

Feature Request

Description
Add --since and --until date filtering options to the /insights command to enable analysis of insights within a specific time period.

Use Case

• Analyze insights from a specific sprint or release cycle
• Compare insights between different time periods
• Review recent insights without seeing all historical data
• Generate period-specific reports for retrospectives

Proposed Syntax

/insights --since 2026-01-01
/insights --until 2026-01-31
/insights --since 2026-01-01 --until 2026-01-31
/insights --since "1 week ago"
/insights --since "last month"

Expected Behavior

• --since: Show only insights created on or after the specified date
• --until: Show only insights created on or before the specified date
• Both options can be combined for a date range
• Support both ISO date format (YYYY-MM-DD) and relative date strings

Additional Context
This would make the /insights command more flexible for users who want to analyze their usage patterns over specific time periods.