Update workflows cmd replaces valid workflow imports

[mnkiefer]

gh aw update removed an unchanged upstream import while updating a sourced workflow.
This caused the recompiled workflow to omit nested imports and their generated steps.

Expected behavior

The unchanged upstream import should remain:

imports:
  - uses: shared/control.md
    with:
      role: orchestrator
      rollout_mode: ${{ vars.CENTRAL_AGENTIC_OPS_DEPENDABOT_MODE || 'preview' }}
      review_repo: ${{ vars.CENTRAL_AGENTIC_OPS_DEPENDABOT_REVIEW_REPO || '' }}

Compilation should traverse:

dependabot.md
  -> shared/control.md
    -> shared/control-precompute.md

The lock file should contain the Precompute control facts step.

Actual behavior

The updated consumer workflow contained:
imports: []

The upstream workflow at the exact pinned source commit still contained the complete shared/control.md import.
The update completed successfully without reporting that the import had been removed.

A sibling workflow, optimization.md, updated from the same package and source commit retained the equivalent shared-control import and compiled correctly.

Suggested fix

The three-way update merge should preserve an import when:

• The import exists in both the merge base and updated upstream workflow.
• The consumer has not explicitly modified or removed it.
  The updater should also avoid silently replacing a non-empty upstream imports collection with [].
  A regression test should cover a prompt-body-only upstream change where nested frontmatter imports remain unchanged.