Committed build/ output drifts from src/ — document the policy and enforce freshness in CI

Problem

All 29 build-capable example plugins commit their build/ directories (~190 tracked files). This appears deliberate — the Playground live demos load plugin files straight from trunk via each _playground/blueprint.json, so build/ must exist in git — but it is documented nowhere, and .gitignore contains an empty, typo'd stanza (# Ingnore "build" folders) suggesting the decision was never finished.

Because nothing verifies src/ and build/ stay in sync, they have drifted. Concrete example: plugins/editable-block-1b8c51 and plugins/interactive-blocks-demos-99def1/src/counter-web-component-99def1 have apiVersion mismatches baked into committed build/block.json. For a teaching repo this is the worst kind of bug: the Playground demo can run different code than the src/ people are reading.

Proposed fix

1. Document the policy in a new CONTRIBUTING.md (the repo has none): build/ is committed on purpose because the Playground blueprints and zip downloads consume trunk directly; contributors must run pnpm build and commit the output. Link it from the root README.
2. One-time rebuild: regenerate every plugin's build/ with its current toolchain so the tree starts fresh.
3. CI freshness check: a workflow job that runs the build and fails the PR if git diff --exit-code 'plugins/*/build' shows changes, with a clear message: "run pnpm build and commit the result". Fail-on-diff (no auto-commit bot) so it works on forks without write access.
4. Fix .gitignore: remove the empty misspelled stanza.

Notes

• The freshness check assumes reproducible builds (same wp-scripts + Node → identical output). Aligning the divergent @wordpress/scripts versions across plugins is a separate normalization issue; when it lands, this check is exactly what forces the accompanying rebuild.
• Hand-maintained asset files in no-build examples (plugins/minimal-block-no-build-e621a6/block.asset.php, plugins/editable-block-1b8c51/src/blocks/basic-editable-block-no-build-1b8c51/block.asset.php) are source, not build output — the check must not flag them.

Out of scope

• Fixing the content-level inconsistencies revealed by the drift (apiVersion, text domains, leftover console.logs) — covered by the plugin normalization sweep issue.

Start by reading the root README, .gitignore, and each plugin's _playground/blueprint.json, then run pnpm build to inspect the current generated output. Add CONTRIBUTING.md and its README link, regenerate committed build/ directories, remove the empty ignore stanza, and add a workflow that fails when git diff detects build changes; done means the tree is fresh and CI reports the pnpm build command clearly.