promote points an (app, channel) at a release id — cross-channel
promotion allowed, missing channel created, manual policy gate stubbed.
rollback moves the pointer to the previous PUBLISHED release in the
channel's publish-order lineage (cross-promoted pointer falls back to the
channel's own latest; at the earliest release it refuses with
rollback.no_previous). Both append a cli-actor audit event and re-persist
db.json; failures follow the P3.4b contract (dotted-code JSON error,
exit 1, store untouched). Acceptance pinned in tests/release_ops.sx;
cli_dispatch reworked off the removed stubs.
