refactor: rename bmad-bmm-product-brief-preview to bmad-product-brief-preview

Drop the redundant bmm prefix from the product brief preview skill folder
to align with the standard naming convention.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/bmm/workflows/1-analysis/bmad-product-brief-preview/SKILL.md

@@ -1,6 +1,7 @@
 ---
-name: bmad-bmm-product-brief-preview
+name: bmad-product-brief-preview
 description: Create or update product briefs through guided or autonomous discovery. Use when the user requests to 'create a product brief', 'help me create a project brief', or 'update my product brief'.
+argument-hint: "[optional --create, --edit, --optimize, --distillate, --inputs, --headless] [brief idea]"
 ---
 
 # Create Product Brief