chore: standardize copyright boilerplate and enforce with CI

[Prachi01Yadav]

Fixes#566
Replaces #568

Summary

This PR standardizes our copyright headers across the whole repo and sets up CI to enforce them going forward. We're now properly reusing the centralized boilerplate checker from kubeflow/testing.

What changed:

• Vendored the checker script: Copied boilerplate.py directly from kubeflow/testing (@60573e4) into hack/boilerplate/. I explicitly matched the vendoring pattern that kubeflow/trainer uses so we don't have to dynamically download the script in CI, and it keeps local dev fast and offline.
• Added CI & Makefile target: Set up a new GitHub Action (code-quality-check.yaml) and a verify-boilerplate make target. It properly handles branch resolution on both pushes and PRs.
• Added missing headers: Slapped the standard, year-less Apache 2.0 template onto 46 files that were missing it entirely. This includes:
  • 22 Python files (even the empty __init__.py ones, to be safe)
  • 13 GitHub Actions workflows
  • A bunch of YAML config files (dependabot, pre-commit, CRDs) and a Dockerfile.
  • (For files with shebangs, I made sure to safely insert the header immediately after line 1).
• Left existing headers alone: Any files that already had a Copyright 2024 or 2025 header were intentionally left completely untouched to keep git blame clean and let the script's normalization logic do its job.

To test locally:
Just run make verify-boilerplate. (I've verified it locally and it passes with 1028 insertions and 0 deletions — strictly additive!).

Checklist:

• Docs included if any changes are user facing

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[tariq-hasan]

/ok-to-test
/retest