Extract the CI validator into a script contributors can run locally

[AnayDhawan]

The validation logic lives inline in a heredoc inside .github/workflows/validate.yml. There is no package.json and no scripts/ directory, so:

• A contributor editing components.json cannot check it before pushing; the only feedback loop is opening a PR and waiting for CI.
• The checks themselves are untestable. CI validator doesn't enforce required fields on fallback_basic entries #26 and CI never checks that an entry's library exists in code_libraries[] #27 added two real rules (required fields on both arrays, library membership in code_libraries[]), and verifying they actually fire meant hand-extracting the heredoc into a temp file and running it against a deliberately broken copy of the JSON.
• The heredoc is invisible to linters and formatters.

Scope

• Move the logic to scripts/validate.py (or .mjs), unchanged in behaviour.
• validate.yml becomes a thin wrapper that runs it.
• Document the local command in CONTRIBUTING.md, in the section that already tells contributors what fields are required.

Note for whoever picks this up

The library-in-code_libraries[] check applies to showpiece[] only. fallback_basic entries point at shadcn and tremor, which are intentionally not code_libraries entries; applying the check to both arrays fails CI on valid data.