fix: make authorship labels idempotent

[haasonsaas]

Summary

• stop removing and re-adding the selected authorship label on every synchronize event
• replace misleading no-trailer human-authored classification with agent-assisted
• keep clearing the legacy human-authored label and document the new semantics
• accept one-character required Maestro trailer values

Verification

• ruby -c .github/scripts/classify-agent-authorship.rb
• classifier smoke checks for agent-assisted, agent-authored, and mixed-authorship outputs
• ruby YAML/JSON parse check for workflow and template metadata
• actionlint .github/workflows/agent-authorship-label.yml

[cursor]

PR Summary

Low Risk
Low risk: changes are limited to PR labeling/classification logic in GitHub Actions and a small Ruby classifier; main risk is mislabeling PRs due to updated heuristics/regex tweaks.

Overview
Updates authorship classification to treat PRs without explicit Maestro trailers as agent-assisted (replacing human-authored), while still labeling mixed-authorship when both trailered and untrailered commits exist.

Tweaks required-trailer regexes to accept one-character values, and makes the labeling workflow idempotent by only removing stale labels and only adding the desired label when it’s not already present; docs/template text are updated to match the new semantics.

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[cursor]

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