Make canonical-evidence gate exempt Dependabot authors

[groupthinking]

Update pr-governance so automated Dependabot dependency PRs are marked neutral instead of failing the canonical issue contract gate.

[linear-code]

GRV-365

[github-actions]

👋 Thanks for opening this issue! It has been automatically labeled for triage.

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• ci: Canonical issue and evidence is permanently unsatisfiable for Dependabot PRs #1419

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