Unify tutorial: documentation and cleanup #769

Summary

Update code comments, remove stale references, and update any documentation that references the old dual-system tutorial approach.

Acceptance Criteria

• TutorialFlow.ts doc comment reflects unified tutorial system
• MainStreetTutorialHints.ts doc comment reflects unified showStep() API
• No stale references to showActionGatedStep() or TUTORIAL_STEPS remain
• No stale references to TUTORIAL_STEP_DEFS remain
• TUTORIAL_STEP_COUNT updated to 13 (was 10)
• TutorialHighlightZone type kept if still used, or removed if no longer needed
• TutorialActionType type kept if still used, or removed if no longer needed
• Code comments explaining the old dual-system approach removed
• If docs/DEVELOPER.md references tutorial architecture, update accordingly

Dependencies

• CG-0MQ8MXYN8008GC22 (merge step definitions — comments must be updated after merge)