[codex] honor control doc slice principles

[noho]

Summary

• Add conditional Slice principle handoff to Phaseflow when control_doc defines Slice cutting principles.
• Require plan and plan review dispatches to include those Slice principles when present.
• Avoid inventing fixed slice counts or thresholds when the control document does not define them.

Why

This makes Slice cutting guidance stable for downstream plan and plan review agents while keeping the rule conditional on project control documents.

Validation

• git diff --check
• Phaseflow frontmatter sanity check