devops: report bundle-size diff on PRs

[nanaf6203-bit]

Summary

No bundle diff on PRs.

Affected area

• Add a workflow that posts bundle-size delta vs main as a PR comment.

Acceptance criteria

• Implement the change described above.
• Add or update unit/e2e tests locking in the new behaviour.
• Update README / docs to reflect the new state.

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Filed automatically as part of the PropChain code-review sweep.