docs: remove "same-repo only" limit from push-to-pull-request-branch, document cross-repo setup

[Copilot]

push-to-pull-request-branch now supports cross-repo usage via target-repo, but the docs still said "same-repo only". This PR updates the reference docs to reflect that and explains the checkout requirement.

Changes

• safe-outputs.md — replace "same-repo only" with "cross-repo supported via \target-repo` when the target repository is checked out"` in the output type summary table
• safe-outputs-pull-requests.md
  • Add target-repo and allowed-repos fields to the push-to-pull-request-branch config block
  • Add Cross-repo usage subsection documenting the mandatory path: checkout requirement and providing a minimal working example:

    checkout:
      - fetch-depth: 0
      - repository: org/target-repo
        path: ./target-repo                  # required — omitting this overwrites the source repo
        github-token: ${{ secrets.CROSS_REPO_PAT }}
        fetch: ["refs/pulls/open/*"]
    
    safe-outputs:
      github-token: ${{ secrets.CROSS_REPO_PAT }}
      push-to-pull-request-branch:
        target-repo: "org/target-repo"
        title-prefix: "[bot] "

• cross-repository.md — add Checkout Requirement for push-to-pull-request-branch callout in the Cross-Repository Safe Outputs section, linking to the new subsection and the existing scheduled-push example

[Copilot]

Pull request overview

Updates reference documentation to reflect that push-to-pull-request-branch supports cross-repository usage via target-repo, and documents the required checkout setup (including the path: requirement).

Changes:

• Update safe outputs summary text to remove “same-repo only” limitation for push-to-pull-request-branch.
• Extend push-to-pull-request-branch docs to include target-repo/allowed-repos and a new cross-repo setup section with an example.
• Add a cross-repository docs callout describing the checkout requirement and linking to the detailed setup.

Show a summary per file

File	Description
docs/src/content/docs/reference/safe-outputs.md	Updates the safe output type summary to reflect cross-repo support for PR branch pushes.
docs/src/content/docs/reference/safe-outputs-pull-requests.md	Documents cross-repo configuration fields and adds a “Cross-repo usage” section + example.
docs/src/content/docs/reference/cross-repository.md	Adds an explicit checkout requirement callout for cross-repo PR-branch pushes and links to examples.

Copilot's findings

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• Files reviewed: 3/3 changed files
• Comments generated: 1

[Copilot]

In the cross-repo example, the org/target-repo checkout uses fetch: ["refs/pulls/open/*"] but does not set fetch-depth: 0. The checkout reference docs recommend fetch-depth: 0 when fetching additional refs to ensure full history/PR details are available; with a shallow clone the push-to-PR flow may not have enough history to inspect or apply changes reliably. Consider adding fetch-depth: 0 to the target repo checkout entry as well.