T60: keep container inputs current

[tomj12k]

Tracked as T60 in the private security repository (Morelitea/security).

The finding, the reasoning, the verification and the review notes live there.
This description is deliberately minimal: the repository is public, and a
detailed write-up of a gap that is still open in a public codebase is itself an
exposure.

Reviewers: look up T60 in Morelitea/security — threat-model.md for the
finding, work-items.md for the acceptance criteria, findings/README.md for
the review history.

Scope of this change: container base inputs, scheduled updates, and the existing image check.

Verified: 304 tests pass on Node 24; updater YAML parses; the selected manifest is a multi-architecture index; the required check name is unchanged.

[tomj12k]

Hosted follow-up: the runtime stage now applies available OS package updates and omits package-manager tooling it does not execute. The exact-image check is rerunning on the amended one-commit head.

The unrelated lease-test synchronization observed in the first run is isolated in #28, which links back here.

[tomj12k]

Rebased onto current main after #23 moved the runtime to Node 26. The conflict resolution preserves that upstream upgrade and pins node:26-alpine to OCI index digest sha256:ef24c5053d50fdc3e4e56eb4e7ddb7861874ab0fdc797046ba897581deb8e868; the runtime package refresh and npm removal remain in place. Exact Node 24 CI-equivalent verification passes: typecheck, 304/304 tests, the command-filter contract, manifest, catalog, and GitHub App registration generation. Local image execution remains unavailable because the Docker daemon is not running, so the required hosted Image builds job is the artifact-level gate.