chore: consolidate legacy specs and plans into superpowers directories

- Move proposal specs from propose/completed/ and propose/stale/ to docs/superpowers/specs/
- Move plans from plans/completed/ to docs/superpowers/plans/
- Add LEGACY FORMAT header to all moved files to prevent agents from using them as patterns
- Create docs/archive/ directory for future use

This consolidates historical specification and planning documents into a
centralized location while marking them as legacy format to distinguish them
from current active work.

Author: HumanBean17