Add `CLAUDE.md` (#42160)

* Add a claude file

* Docs: route CLAUDE.md to a tool-agnostic AGENTS.md

Rename the agent guide to AGENTS.md (the emerging cross-tool convention
used by Copilot, Cursor, Codex, etc.) and leave CLAUDE.md as a one-line
`@AGENTS.md` pointer so Claude Code keeps working without duplicating
content. Heading made tool-agnostic.

diff --git a/AGENTS.md b/AGENTS.md
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+# AGENTS.md
+
+Guidance for AI coding agents working in this repository. Tool-agnostic; `CLAUDE.md` points here.
+
+Bootstrap v6.0.0-alpha1 — CSS/JS framework.
+Sass source in `scss/`, JS source in `js/src/`, docs site in `site/` (Astro 5 + MDX).
+
+## Quick commands
+
+- `npm start` — dev server (watch + Astro on port 9001)
+- `npm run dist` — full CSS + JS build
+- `npm run css` — compile, prefix, minify CSS
+- `npm run js` — compile + minify JS
+- `npm run css-lint` / `npm run js-lint` — lint
+- `npm run docs-build` — build docs site
+- `npm test` — full test suite (lint + dist + tests + docs)
+
+## SCSS conventions
+
+- Variable naming: `$component-state-property-size` (e.g., `$nav-link-disabled-color`)
+- Module system: `@use` / `@forward` only — never `@import`
+- Token system: component tokens are SCSS maps of CSS custom properties, merged via `defaults()`, output via `@include tokens($map)` on the component root selector
+- Disallowed: raw `border-radius` / `transition` properties (use mixins), `lighten()` / `darken()`, `border: none` / `outline: none`
+- Order: `@use` → `@forward` → `$variables` → `--custom-properties` → declarations → rules
+- Variables use `!default` (except locals)
+- CSS layers: `colors, theme, config, root, reboot, layout, content, forms, components, custom, helpers, utilities`
+- Logical properties: prefer `padding-inline-start` over `padding-left`, etc.
+- Modern CSS: `color-mix()`, `light-dark()`, range media queries (`@media (width >= 1024px)`)
+- Doc markers: `// scss-docs-start name` / `// scss-docs-end name` for docs extraction
+
+## JavaScript conventions
+
+- ESM-only, no semicolons, 2-space indent
+- Components extend `BaseComponent` (which extends `Config`)
+- Constants: `NAME`, `DATA_KEY`, `EVENT_KEY`, `VERSION`
+- DOM utilities via `dom/event-handler.js`, `dom/selector-engine.js`, `dom/manipulator.js`
+- Floating UI for positioning (dropdown, tooltip, popover)
+- Tests: Jasmine + Karma, specs in `js/tests/unit/*.spec.js`
+
+## Docs conventions
+
+- Astro 5 + MDX in `site/src/content/docs/`
+- Frontmatter schema: `title` (required), `description` (required), `toc`, `aliases`, `added`, `mdn`, `reference`, etc.
+- Shortcodes: `Example`, `Callout`, `Code`, `Details`
+- Internal links: `[[docsref:/path/]]`
+
+## Formatting
+
+- 2 spaces, UTF-8, LF line endings, trim trailing whitespace, final newline
+- Preserve smart punctuation (curly apostrophes U+2019, curly quotes) — never replace with ASCII equivalents
+- Use `StrReplace` for targeted edits; avoid full-file rewrites on MDX files to protect typography
+
+## Workflow
+
+- Validate after changes: `npm run css-lint`, `npm run js-lint`, `npm run dist`, `npm run docs-build`
+- Keep config files in sync: `config.yml`, `site/src/libs/config.ts` (Zod schema), `build/rollup.config.mjs`, `build/generate-sri.mjs`, `package.json`
+- For rebases/conflicts: favor base branch structure, layer feature changes on top
+
+## Do not edit
+
+Built output in `dist/`, `_site/`, `js/dist/` — these are generated.

diff --git a/CLAUDE.md b/CLAUDE.md
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+@AGENTS.md