publish-apk: cap the backups it creates

Four publishes in one evening left 448MB of .bak copies of a 112MB APK, in a
public web directory. A backup policy with no retention becomes the problem it
was guarding against.

Keeps the two most recent, configurable with --keep. Prunes before copying, so
a failed publish doesn't skip the pruning as well.

The first version of the test gave all four fixture backups the same mtime, so
'ls -t' ordered them arbitrarily and 'keeps the most recent' proved nothing --
it only failed because the arbitrary order happened to go the wrong way. Worth
recording: a test that passes for the wrong reason is the same class of
problem as the dead code these guards exist to catch.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

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