Add per-project filtering to /insights command

Problem

The /insights command analyzes all sessions globally, mixing data from unrelated projects into a single report. When working across multiple projects (different languages, different domains), the insights become unfocused and hard to act on.

For example, my report mixed Obsidian vault management, Flutter UI work, PHP web development, and WSL debugging into one analysis. The suggestions and friction points from one project aren't relevant when I'm working in another.

Proposed Solution

Add project-scoping to /insights, e.g.:

/insights              # Current project only (based on cwd)
/insights --all        # All projects (current behavior)
/insights --project /path/to/project

The default should probably be current project, since that's the most actionable context when you're actively working.

Why This Matters

• Insights are most useful when they reflect patterns in the project you're currently working on
• Per-project friction analysis and suggestions would be directly actionable
• CLAUDE.md suggestions would actually match the project's stack and conventions
• Cross-project noise makes the report harder to parse and less trustworthy