feat(webpack): add webpackFilesystemCache for production builds

[ak-seek]

Summary

• Adds webpackFilesystemCache to sku.config so consumers can opt in to Webpack 5 filesystem cache for sku build / sku build-ssr and CI.

• Accepts either a mode ('development' | 'always') or an options object exposing Webpack's advanced cache options:

  webpackFilesystemCache: {
    mode: 'always',
    compression: 'gzip',
    maxAge: 1000 * 60 * 60 * 24 * 7,
    buildDependencies: ['./tsconfig.json'],
  }

• Sku always invalidates the cache on sku.config.* and on the installed sku version; consumer buildDependencies are appended.

• Mode override via SKU_WEBPACK_FILESYSTEM_CACHE=development|always; full disable via SKU_DISABLE_CACHE.

• Default behaviour is unchanged ('development' → cache only for the dev server).

Changeset

Includes .changeset/webpack-filesystem-cache-config.md per CONTRIBUTING.

Docs

• docs/docs/configuration.md
• docs/docs/extra-features.md (incl. example, BuildKit cache mount note)

Test plan

• pnpm --filter sku lint:tsc
• pnpm lint:tsc
• pnpm lint:eslint
• pnpm lint:prettier
• pnpm test (CI)

[changeset-bot]

🦋 Changeset detected

Latest commit: 0ee3d87

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
sku	Minor

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[ak-seek]

Thanks for the feedback (see here). Closing this in favour of testing the approach via dangerouslySetWebpackConfig (with BuildKit cache mounts on node_modules/.cache/webpack) in our own apps first. Once we have data showing it actually helps cold and warm builds materially, we can revisit upstreaming it.