Support target-only checkout for MultiRepoOps sidecar workflows (#56504)

* Initial plan

* Add target-only checkout support via checkout enabled: false

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* Apply skipDefaultCheckout signal from imported checkout configs

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* Replace enabled field with permissions.contents: none signal for target-only checkout

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* Document target-only checkout in instructions and pattern docs

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* Add ADR draft for target-only checkout

* Gate safe-outputs checkout on CheckoutSkipDefault; dedupe permissions detection; warn on empty target-only checkout

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>