fix(renovate): exempt the automerge tier from minimumReleaseAge

The automerge tier does not wait out minimumReleaseAge: pin and digest do not change what is installed and lockFileMaintenance is constraint-bound, so there is no version decision to soak. Under Renovate 43's minimumReleaseAgeBehaviour=timestamp-required, a release without a releaseTimestamp (docker digests, custom datasources) is pending FOREVER, so the global 7-day age held renovate/stability-days at pending on every tier MR and platformAutomerge could never fire -- the tier shipped 2026-08-06 and never merged one MR. Measured 2026-08-10 on manifold!63 (merged) and reference!48 (merged).

The catalog soak is unaffected: the rule scopes to update types, and the shipping-class rules below still override automerge back to human review where they match. Lab-verified (renovate-lab.sh dry-run with the rule overlaid): tier branches carry no stability status at all; non-tier branches keep theirs.