feat: add PR size labeler workflow

[gTanusri]

Summary

• Added a dedicated GitHub Actions workflow to label pull requests by diff size.
• Used pascalgn/size-label-action@v0.5.7 with thresholds for size/XS through size/XXL.
• Added a preflight step to create missing size labels automatically.
• Ignored lock files and generated output directories from size calculations.
• Updated PR label inheritance so size/ labels are treated as workflow-managed labels.

Why

Large pull requests are harder to review and triage quickly. Automatic size labels give maintainers an immediate signal about review effort without opening the full diff.

Validation

• Ran git diff --cached --check before committing.
• Checked the changed workflow files for trailing whitespace.

Related Issue

Closes #5283

[vercel]

@servis is attempting to deploy a commit to the sandeepvashishtha's projects Team on Vercel.

A member of the Team first needs to authorize it.

[priyanshirai231-spec]

Hello! I'm a contributor for GSSoC '26 and I'm very interested in resolving this issue. Could you please assign it to me? Thank you!