Align create-plan with current plan-tooling workflow

[graysurf]

Align create-plan with current plan-tooling workflow

Summary

Align create-plan with the current plan-tooling CLI contract so generated plans are checked for parsing, batching, and PR-splitting executability instead of relying on validate alone.

Changes

• Add the mandatory plan-tooling to-json, plan-tooling batches, and plan-tooling split-prs validation pass to the create-plan workflow.
• Document the default metadata-first auto grouping flow plus deterministic group and per-sprint validation modes.
• Add regression tests for grouping metadata and the no cross-sprint execution-parallelism rule.

Testing

• scripts/check.sh --all (pass)

Risk / Notes

• Keep create-plan lighter than create-plan-rigorous; this change adds CLI compatibility guidance without copying the full sizing rubric or subagent-review flow.