Add date/session filtering to /insights command

Problem

The /insights command currently analyzes all sessions from the last 30 days with no way to filter or narrow the scope. For users whose projects evolve significantly over that period — different phases, pivots in workflow, or even different sub-projects — the older sessions add noise that clouds the analysis.

In my case, early sessions were dominated by MCP tooling debugging and scene-building, while recent sessions are pure game design and architecture work. The insights report blends these into one narrative, making the recommendations less actionable.

Proposed Solution

Add optional filtering flags to /insights:

• Session count: /insights --last 28 — analyze only the N most recent sessions
• Date range: /insights --since 2026-02-10 — analyze sessions from a specific date forward
• Date window: /insights --from 2026-02-01 --to 2026-02-15 — analyze a specific date range

Any of these would be a significant improvement over the current fixed 30-day window.

Why This Matters

• Projects evolve — what was relevant 3 weeks ago may not reflect current workflows
• Users with high session counts (100+/month) accumulate a lot of stale context
• Filtering would make the friction analysis and suggestions much more targeted and actionable
• The data is already timestamped per-session, so filtering should be feasible on the existing data

Current Workaround

Manually moving old session transcript files out of ~/.claude/projects/ before running /insights, then moving them back. This works but is fragile and tedious.