Rewrite SKILL.md with complete template documentation

Document all 17 template slides with semantic shape maps and strict template-only workflow.

## Details
- Document each of the 17 template slide layouts with their semantic shape maps
- Include visual descriptions and expected content for each semantic key
- Define strict template-only workflow (no creating shapes from scratch)
- Provide examples of how to use inventory output to fill slides
- Remove references to old free-form shape creation workflow

## Part of: PPTX Skill v6.0 Overhaul