Build dist for all npm dependency updates

[jeffwidman]

Rebuild the checked-in action bundle for every npm Dependabot update, including indirect dependencies and development build tooling.

The existing workflow only runs for direct:production updates. That misses updates such as undici in #734, even though the transitive dependency is included in dist/index.js. Dependency classification is not a reliable proxy for bundle impact; development tooling such as esbuild can also change generated output.

The workflow now uses the build output as the selector:

• Run for every npm_and_yarn Dependabot PR.
• Stage the rebuilt dist/ directory.
• Exit without a commit or push when dist/ is unchanged.
• Commit and push only when the bundle actually changed.

The explicit staged-diff check also avoids suppressing genuine git commit errors with git commit ... || exit 0.

This matches the approach used by github/dependabot-action.

Validation:

• Workflow YAML parses successfully.
• A clean rebuild from main produces no staged change and takes the no-op path.
• Rebuilding PR build(deps): bump undici from 6.27.0 to 6.28.1 #734 detects the expected dist/index.js change and takes the commit path.
• npm run typecheck
• npm run lint
• git diff --check