Fix Evergreen safe_outputs patch application by pre-checking out target PR branch

[Copilot]

The safe_outputs job in Evergreen was failing when push_to_pull_request_branch attempted to apply an empty patch (git am), caused by generating patch content from the wrong baseline. This updates Evergreen so agent work starts on the selected PR branch directly.

• Root cause alignment in workflow pre-flight

  • Updated the PR selection Python step in .github/workflows/evergreen.md to switch to the selected head_branch before agent execution.
  • Uses a local tracking branch to ensure push tooling operates on the correct branch state.

• Agent prompt contract update

  • Revised instructions to reflect that branch checkout is already handled in pre-flight, reducing ambiguity and duplicate checkout behavior in the agent phase.

• Compiled workflow update

  • Regenerated .github/workflows/evergreen.lock.yml from the updated source workflow definition.

branch = selected["head_branch"]
subprocess.check_call(["git", "checkout", "-B", branch, f"origin/{branch}"])
subprocess.check_call(["git", "branch", "--set-upstream-to", f"origin/{branch}", branch])