Refine generated update validation

[patchstackdave]

Summary

• Allow the generated-update merge job to read linked workflow-run metadata used by its existing check gate.
• Validate the proposal author using the canonical API login.
• Keep the existing file, signature, provenance, and CI requirements unchanged.

Verification

• Workflow YAML parsed locally.
• Embedded Bash passed syntax validation.
• git diff --check passed.
• GitHub reports the commit signature as verified.

[coderbuds]

Workflow validation logic neatly streamlined and improved for bot PR verification.

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[patchstackdave]

/review