The PRD workflow step-02 was refactored to be discovery-only with
forward references to steps 2b and 2c, but those files were never
created. This left a gap where the workflow jumped from classification
directly to success criteria with no executive summary generation.
- Add step-02b-vision.md: collaborative vision/differentiator discovery
- Add step-02c-executive-summary.md: generate and append exec summary
- Update step-02 nextStepFile to chain through 02b instead of skipping to 03
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