claude agents: add workspace scoping to filter sessions by directory

Summary

claude agents shows all background sessions globally. There's no way to scope the view to sessions started from a specific workspace or directory.

When working across multiple projects, the session list gets noisy. It would be helpful to filter sessions by the directory they were started in.

Proposed behavior

Add a flag (e.g. --cwd or --workspace) to filter sessions by their working directory:

# Show only sessions started from the current directory
claude agents --cwd .

# Show only sessions started from a specific path
claude agents --cwd ~/code/my-project

Alternatively, claude agents could default to showing sessions from the current directory, with a --all flag to show everything.

Current behavior

claude agents always shows all background sessions regardless of which directory it's run from.