[FEATURE] Scope /insights to current project directory

Problem

/insights aggregates session data across all projects globally. When running /insights from a specific project directory (e.g., /dev/basic), it includes data from unrelated projects (e.g., a personal chess project), making the insights less relevant and harder to act on.

Proposed Solution

Scope /insights to only analyze sessions associated with the current project directory by default. This could work by:

1. Filtering sessions based on the working directory they were started in
2. Adding an optional --global flag to preserve the current cross-project behavior when desired

Why This Matters

• Developers working across multiple projects get diluted insights that mix unrelated contexts
• Project-specific patterns and recommendations are lost in cross-project noise
• Other features (MCP servers, plugins, CLAUDE.md) already respect project-level scoping — /insights should too