fix: tolerate authorship label delete races

[haasonsaas]

Summary

• handle the missed Bugbot feedback from fix: make authorship labels idempotent #14
• keep stale authorship-label deletes strict for real failures, but tolerate a 404 when a concurrent run already deleted the label
• preserve idempotent label application without introducing noisy race failures

Verification

• actionlint .github/workflows/agent-authorship-label.yml
• ruby -c .github/scripts/classify-agent-authorship.rb

[cursor]

PR Summary

Low Risk
Low risk workflow-only change that makes label cleanup idempotent by tolerating expected 404/not-found responses from concurrent runs while still surfacing other API failures.

Overview
Makes the agent-authorship-label workflow resilient to concurrent executions by wrapping label deletion in delete_label_if_present, which treats 404/"not found" responses as non-fatal.

This keeps removal of stale authorship labels strict for real errors while avoiding flaky failures when another run already deleted the label.

^{Reviewed by Cursor Bugbot for commit 25ef152. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is ON, but it could not run because the branch was deleted or merged before autofix could start.}

^{Reviewed by Cursor Bugbot for commit 25ef152. Configure here.}