ci(images): publish chronologically-orderable master-<epoch>-<sha> tags

The existing master push pipeline produces `master` (rolling) and
`sha-<short>` tags. Neither is orderable by build time, so downstream
GitOps that want to auto-bump to the newest master build (e.g. Flux
ImagePolicy) can't pick the latest from the tag list — alphabetical
sort over hex shas is effectively random, and the rolling `master`
tag can't be referenced as an immutable bump target.

Add a third tag of the form `master-<epoch>-<sha>` (Unix epoch in
seconds + short sha), gated on default-branch pushes via metadata-
action's `is_default_branch` predicate. The sha is retained for
traceability; the epoch makes the tags numerically orderable, so a
Flux ImagePolicy like

  filterTags:
    pattern: '^master-(?P<ts>[0-9]+)-[a-f0-9]+$'
    extract: '$ts'
  policy:
    numerical:
      order: asc

will reliably bump to the newest master build.

Applied to both image_build.yml (OCI labels stay consistent) and
image_merge.yml (the actual tag publisher via buildx imagetools).