feat(cmd): add markdown file support to deck new command

[Songmu]

Add optional markdown filename argument to deck new command that automatically creates or updates the file with frontmatter containing title and presentationId.

Changes:

• Accept optional markdown filename as first argument in cmd/new.go
• Create file and parent directories if they don't exist
• Parse and preserve existing frontmatter fields when updating
• Add or update title and presentationId fields

Usage:
deck new presentation.md --title "My Presentation"

This simplifies the workflow by combining presentation creation and markdown file setup in a single command.

[k1LoW]

@Songmu Thank you!