[Feature] Sprint Goal Dependency Audit for /sprint-plan skill

## Summary Add Phase 1b: Sprint Goal Dependency Audit step to check prerequisites before making the sprint plan to prevent unrealistic goals. ## Type - [X] Improvement to existing component (/sprint-plan) ## Problem / Motivation I've face this problem during the development of my game. The sprint plan were include a playtest from author, but the sprint itself contains only logic code. The scene required to run the test were not exist yet. This resulting in some stories can't be fulfilled and need to defer to the following sprint instead. No big deal - but this kind of surprise is annoying for me. ## Proposed Solution The improvement is already made, to summarize what Phase 1b do: Step 1 — State the sprint goal explicitly. Step 2 — Identify what the sprint goal requires to be demonstrable. Step 3 — Check prerequisites exist. (scene files) Step 4 — Surface missing prerequisites. Step 5 — Proceed to Phase 2 with prerequisites resolved. If you want a pull request, just tell me. 🙂