Add date range filtering to /insights command

Feature Request

The /insights command currently analyzes all sessions in the data set with no way to filter by date range. It would be useful to scope the analysis to a specific time period.

Proposed Usage

/insights --since 7d        # last 7 days
/insights --since 2026-02-01 --until 2026-02-14  # specific range
/insights --last-week        # shorthand for last 7 days

Use Case

When running /insights frequently (e.g., weekly), the full historical analysis becomes less actionable over time. Being able to focus on recent sessions would make it easier to:

• Track whether friction patterns are improving week-over-week
• Get targeted suggestions based on recent work rather than all-time trends
• Generate shorter, more focused reports for team standups or retros

Current Workaround

None — the command always processes the full session history.