Apply reviewed labels when opening contributions

[hamzamerzic]

Summary

• accept at most two owner-reviewed PR labels from a prepared contribution
• resolve labels against the target repository's current taxonomy before applying them
• preserve the PR as successfully open when labels are missing, permissions are insufficient, or GitHub label lookup times out
• record requested, applied, and missing labels so Contribute reports the real outcome

Companion

mobius-os/app-contribute#13 shows the proposed labels and related-work evidence in the review UI.

Verification

• all 88 GitHub route tests
• transport-exception, permission, missing-label, duplicate, and visible-two regressions
• git diff --check

[hamzamerzic]

Superseded by #138, which keeps the best-effort non-fatal label path, adds end-to-end transport-failure coverage, and now matches the exact pair shown in Contribute review. Closing this duplicate so there is one reviewed landing path.