Fix Dependabot secret resolution for GitHub Packages registry (#622)

Fix Dependabot secret resolution for GitHub Packages registry (#622)

* Initial plan

* fix: use DEPENDABOT_GITHUB_PACKAGES_TOKEN instead of GITHUB_TOKEN in dependabot.yml

GITHUB_TOKEN is not available as a secret in the Dependabot context
because Dependabot version updates run on their own infrastructure,
not on GitHub Actions runners. Replace with a PAT stored as a
Dependabot secret.

Update documentation in .github/workflows/README.md and
docs/development/workflows.md to reflect the PAT approach as the
current method with setup instructions.

Co-authored-by: metanull <20630081+metanull@users.noreply.github.com>
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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: metanull <20630081+metanull@users.noreply.github.com>