docs: README styleguide section — rules, live URL, app usage

README.md

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+
+## Styleguide
+
+**vault1984.css** is the single global stylesheet for all vault1984 web surfaces — marketing site (`vault1984-web`) and the app UI (`vault1984`).
+
+- Live: https://vault1984.com/styleguide.html
+- Source: `vault1984-web/vault1984.css`
+
+### Rules (no exceptions)
+1. **One stylesheet.** `vault1984.css` only. No Tailwind, no inline styles, no `<style>` blocks.
+2. **One rule per class.** If you need a variant, add a modifier class (e.g. `.card.gold`), not a new inline style.
+3. **One width.** `--width: 1280px` via `.container`. Never hardcode a max-width anywhere else.
+4. **One padding.** `--pad: 2rem` via `.container`. Never hardcode horizontal padding.
+5. **CSS variables for everything.** Colors, spacing, radius, fonts — all via `var(--*)`.
+
+### To use in vault1984 app
+Copy or symlink `vault1984.css` into `vault1984/web/` and embed it. The token set (colors, fonts, radius) is shared — app UI should feel identical to the marketing site.