/insights: add project-level and date-range filtering

Problem

/insights aggregates ALL sessions across all project directories, mixing work and personal usage into a single report. This makes the report unsuitable for sharing in a professional context.

Current behavior

Running /insights in any project directory produces a single report combining sessions from all project_path values in ~/.claude/usage-data/session-meta/. For example, my Azure infrastructure work (C:\repos) and homelab debugging sessions (D:\personal\repos) are combined into one report.

Requested behavior

Add filtering flags to scope the report:

# Filter by project directory (prefix match)
/insights --project C:\repos

# Exclude specific projects
/insights --exclude D:\personal

# Date range
/insights --since 2026-02-15 --until 2026-03-01

Why this matters

• Shareability: Work reports shouldn't include personal homelab debugging sessions
• Accuracy: Mixed-context reports dilute the analysis — friction patterns, tool usage, and project areas are muddled
• Weekly reviews: Running filtered reports on a cadence (weekly, per-sprint) is useful for team retrospectives

The data already supports this

Each session-meta/*.json already contains project_path and start_time fields. The filtering would only need to happen at the session-selection step before facet analysis begins.