Add html skill for web pages and email templates

[ZihengXiong]

Adds a new html skill covering two distinct use cases — static web pages
and HTML email templates — which have very different rules and constraints.

Files

• SKILL.md: entry point with quick-reference table, web-vs-email decision
  guide, ready-to-use boilerplate for both, and key rules at a glance
• reference.md: full web reference — semantic HTML, responsive layout
  (Flexbox / Grid), CSS custom properties, accessibility (WCAG 2.1), forms,
  common patterns (hero, card grid, modal, hamburger menu, toast), performance
  (srcset / WebP / Core Web Vitals), and common pitfalls
• email.md: email-specific guide covering client compatibility matrix
  (Gmail, Outlook, Apple Mail, Yahoo), table-based layout, inline CSS
  workflow, typography, images, responsive fluid layout, dark mode,
  bulletproof CTA buttons, a complete 200-line marketing template, and a
  pre-send testing checklist
• scripts/inline_css.py: inlines <style> CSS into element style=""
  attributes before sending (required for Gmail / Yahoo compatibility);
  preserves @media queries in <style>; depends on premailer
  (pip install premailer)