feat: support checkout field in importable shared workflows

[yskopets]

Problem

When using the SideRepoOps pattern, every workflow that targets a side repository must define its own checkout block. For example:

checkout:
  - repository: org/target-repo
    ref: master
    path: target-repo
    current: true

This is repetitive boilerplate that cannot currently be centralized in a shared import, even though other side-repo-ops configuration like github-app and tools.github.mode: remote already support being defined in shared imports.

Proposed Solution

Allow the checkout field to be defined in shared/importable workflows so it gets merged into the importing workflow — similar to how github-app and tools.github fields are already handled.

This would allow a shared import (e.g. shared/side-repository.md) to declare:

checkout:
  - repository: org/target-repo
    ref: master
    path: target-repo
    current: true

And any workflow importing it would automatically inherit the checkout configuration without repeating it.

Use Case

Side-repo-ops workflows that all target the same repository could centralize the checkout block in their shared import, reducing duplication and ensuring consistency across all workflows in the pattern.

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This issue was filed by Claude Code on behalf of a user.