Introduce prepare-release for the deterministic parts of preparing a
release. Update AGENTS.md to use that, rather than using an LLM for
deterministic steps.
finalize-release now pushes the release commit and tag after signing, and
prepares a release announcement mail.
Assisted-by: Claude:claude-opus-4-6i
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
