Add the missing che-pptx-mcp usage skill

[kiki830621]

Refs #159

Summary

• ship the missing che-pptx-mcp plugin skill with concrete Direct, create, and edit workflows
• document exact indexing, shape-kind, EMU, autosave, lifecycle, and unsupported PDF/render boundaries
• link the skill from plugin docs and bump the shell distribution to 0.1.3 without changing binary 0.1.0

Verification

• skill RED/GREEN behavioral scenarios passed
• skill structure validation passed
• tool/parameter claims checked against the published server surface
• requirements, regression/packaging, claim-parity, and adversarial reviews passed after four correction rounds
• git diff --check passed

Checklist

• Diagnose
• Implement (5 commits)
• Verify-gated PASS
• No automatic GitHub closing trailer

After merge, use the explicit IDD closing workflow for issue 159.