Yeti phase 0: repo transition and tooling skeleton

[joeworkman]

What this changes

This is the first Yeti pull request. It turns the Foundation for Sites 6 repository into the Yeti repository: the 6.x tree
is removed (6.x continues on v6), and the source skeleton, tooling, tests, CI, and rewritten project files land in its
place. No CSS beyond the cascade layer declaration exists yet; that is phase 1.

Removed. Sass, gulp, jQuery plugins, Babel, Rollup, webpack, mocha, BrowserStack, the customizer, the Panini docs,
husky, commitlint, and every config file that served them. Everything is preserved on v6 and in history.

Added.

• src/yeti.css importing src/layers.css, which declares @layer yeti.reset, yeti.base, yeti.layouts, yeti.components, yeti.utilities;. The unbuilt source tree loads directly in a browser.
• schema/manifest.schema.json, the contract every layout and component will be authored against.
• bin/ tooling in plain Node with three dev dependencies (Playwright, axe, parse5): validate (manifests, examples, the
  spacing-ownership rule, the layer contract), build (concatenation only, source comments preserved), gen-docs (Markdown
  pages in the foundationcss.com front-matter convention), and a release skeleton.
• 74 node:test tests and a Playwright smoke test in Chromium, Firefox, and WebKit that asserts the layer order from CSSOM
  and runs axe.
• A two-job GitHub Actions workflow with no secrets and no matrix. This PR is its first real run.
• README, CONTRIBUTING, SECURITY, CODE_OF_CONDUCT, issue and PR templates rewritten for Yeti.
• License changed to FSL-1.1-MIT (Functional Source License 1.1, MIT after two years), Licensor Aspect Services, LLC.
  Foundation for Sites 6 remains MIT on v6.
• Node 24 pinned; tabs for JS, CSS, and HTML; JSON and YAML at two spaces.

Version. develop moves to 7.0.0-alpha.0. master stays at 6.9.0 until 7.0.0 is real.

After merging: publish the announcement issue, rename the repository to foundation/yeti, update the required status
checks on develop to Validate, test, build and Browser smoke, and patch the README's announcement link with the issue
number.

Checklist

• npm test passes locally (validate, tooling tests, browser smoke)
• Any new layout or component lands with its manifest.json, example.html, and generated docs page in this PR (none
  in this phase)
• No new dependencies beyond the three the architecture allows
• Component CSS sets no outer margin on its identity selector; layouts own spacing (enforced by bin/validate.js)
• Targets develop

🤖 Generated with Claude Code

https://claude.ai/code/session_01EN3wewLS1sGobTkT9NSyE3

The two footer lines are the standard attribution for work done in this session. Drop them if you'd rather the PR carry no
tool attribution, the same way the commits don't.

If you want me to push the branch and open this PR for you, say so; otherwise it's ready to paste.