fix: prefer manually added team labels over topology lookup

[sleepytanya]

Summary

• Skip topology assignment in add-team-label when the PR already has a team-* or external-contributor label
• Keeps manually applied team labels from being layered over by the author's topology match
• Logs both skip and assign paths for clearer Actions output
  Fixes: MCRM-72

---

Note

Low Risk
Narrow change to PR labeling automation in GitHub Actions; no application runtime or security-sensitive logic.

Overview
The add-team-label composite action no longer overwrites PRs that already have a team-* or external-contributor label.

A new first step reads current labels via gh pr view; if one of those labels is present, topology lookup and label application are skipped (with log lines for skip vs assign). Get team label and Add team label run only when no such label exists. The action description and CHANGELOG document the behavior.

^{Reviewed by Cursor Bugbot for commit a1d3371. Bugbot is set up for automated code reviews on this repo. Configure here.}