Completion and manpage linking trusted any tap whose name looks official, so a tap named homebrew/foo cloned from a custom remote had its shell completions linked automatically even though the trust checks would not run its Ruby. Add Tap#official_git_checkout?, which requires the checkout's actual Git origin to be the canonical remote, and use it where completions are linked, unlinked and counted, so only canonical official checkouts get automatic completion trust.

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

GPT-6 Astra and Claude Code (Fable 5.1) drafted the implementation and tests; I reviewed the diff, verified the new test fails without the change and the updated specs pass with it, and ran brew lgtm --online plus targeted specs.