Share a single author date across all automatic commits

Export `GIT_AUTHOR_DATE` once, right after checkout: the author date of
the checked out commit. All automatic commits (changelog, `revision`,
`project.build.outputTimestamp`) and the timestamp property now share
this value, so re-runs of the workflow regenerate commits with
identical author dates.

Assisted-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A4jy56QVeduMJcpkryJQpG