feat: adversary feature

[ematipico]

Factory Adversary

• Added an opt-in Blue/Purple adversarial PR workflow.
• Blue independently implements the requested change from the exact base commit.
• Purple compares Red and Blue using qualification and quality criteria.
• When Purple selects Blue, Factory pushes the alternative branch and opens a draft PR.
• The draft PR follows the caller repository's PR template.
• The original PR receives Purple's summary and a link to the alternative.
• Other verdicts produce only a concise decision comment.
• Added isolated sandboxes, temporary R2 patch storage, checks, configuration, documentation, and tests.