Document CLI improvement plan and student scope

[dlq]

Summary

• Add a phased CBRAIN CLI improvement backlog in plan.md
• Add a scoped 3-month summer student project brief
• Document developer setup, Ruff/pre-commit workflow, and capture tests in README

Notes

The student scope intentionally references plan.md as the larger technical backlog and keeps the summer expectations focused on correctness fixes, guardrails, and one architecture slice.

Validation

• Documentation-only change
• Reviewed git diff locally
• Worktree clean before PR creation