Add date range options to /insights command

Feature Request

The /insights command currently auto-selects a recent window of sessions (roughly the last ~8 days) with no way to customize the time period.

Requested capability

Add date range options to /insights so users can control the analysis window. For example:

• /insights --since 2026-03-01 — analyze sessions from a specific start date
• /insights --since 2026-03-01 --until 2026-03-31 — analyze a specific date range
• /insights --days 30 — analyze the last N days

Why this matters

Power users running many sessions per day (50+ per week) want to:

• Generate monthly or quarterly reports for retrospectives
• Compare usage patterns across different time periods
• Analyze sessions from a specific project phase or sprint
• Get insights beyond the default ~1 week window

Current behavior

/insights analyzes a fixed recent window (e.g., 47 of 277 total sessions) with no parameters to extend or customize the range.

Expected behavior

Users can specify a date range or lookback period, and /insights filters sessions accordingly before generating the report.