Update writes every mutable column of agent_runs except checkpoint.
That exclusion is intentional: PGCheckpointer.Save and ClearCheckpoint
are the only paths that touch the column, so a status commit cannot
overwrite an in-flight checkpoint saved between Load and Update.
Surface the rule on the Update method so future readers do not patch
in a checkpoint write thinking it was an oversight.
Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>
bfa1c21723