docs: Add command iteration requirements analysis

Planning document for Phase 1 implementation:
- Analyzes all 16 MetaSpec commands
- Identifies which need iteration support
- Provides decision criteria and rationale
- Recommends phased implementation approach

Key findings:
- High priority: analyze, clarify commands (SDS + SDD)
- Already done: sds.checklist (v0.1.3+)
- Not needed: specify, implement, constitution commands

This serves as the roadmap for future iterations of the
iteration-aware design implementation.

Author: NeilJo-GY