feat: add HTML-first slide skill

[tvgh]

Summary

• add a standalone codex-html-slides skill that produces self-contained HTML presentations without full-slide raster outputs
• include a reusable 1600×900 presentation runtime, 12 native HTML style recipes, and guidance for translating reference-image aesthetics into DOM/CSS/inline SVG
• add deterministic deck creation and validation scripts
• package the new skill in release ZIPs and document installation in both READMEs

User-visible changes

Users can now create offline-ready HTML slide decks whose page elements remain editable in source. The workflow intentionally avoids image-generation backends, slide screenshots, PPTX, and PDF output while preserving image-quality composition through native browser techniques.

Changelog

• Updated CHANGELOG.md
• Not needed: internal-only change

The changelog entry includes the required (#88) PR reference.

Verification

• quick_validate.py skills/codex-html-slides
• Python bytecode compilation for both bundled scripts
• strict create/validate round-trip for all 12 themes
• negative validation test for external script rejection
• in-app browser QA at 1600×900, 1366×768, and 390×844, including overflow, notes controls, and console errors
• Ruby YAML parse and bash -n check for the changed release workflow block
• release ZIP smoke test confirming the new skill files are packaged