Why: Backend #171 HEAD advanced with must_change e2e fix; keep Dev gitlink in sync and mark U4 specs started.
Evidence: Backend go test store/server/app green at 47463a9
Impact: Dev U4 branch tracks Backend PR #171 tip.
Refs: docs/svc-deploy/u4-one-time-login-key.md; Backend pulls/171
Co-authored-by: Cursor <cursoragent@cursor.com>
Why: wire Dev to the Backend feature that delivers OpenExisting and
one-time login keys for the approved deploy proposal.
Evidence: SELF Backend tests; gitlink 1ba7225.
Impact: Backend gitlink + docs; runtime via new binary.
Refs: .task/feature-svc-deploy-u4-login-key.md
Co-authored-by: Cursor <cursoragent@cursor.com>