feat: add standalone editable HTML PPT skill

[Anna-YC]

Summary

• Add skills/editable-html-ppt as an independent companion skill for editable standalone HTML presentations.
• Include content-to-deck guidance, three visual systems, speaker notes/presenter mode, and two PPTX export modes.
• Add browser editing for text, inserted objects, visual layers, structural borders, undo/redo, keyboard controls, and a compact movable toolbar.
• Vendor the browser-side export dependencies so the skill remains self-contained.

Validation

• Validated the base deck and a generated deck for editing, visual-layer selection, undo/redo, both PPTX exports, presenter controls, and timer behavior.
• Checked the injected editor runtime syntax.