Document AgentRun.Update checkpoint write split

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>

pkg/coredata/agent_run.go

@@ -346,6 +346,11 @@ RETURNING
 	return nil
 }
 
+// Update intentionally does not write the checkpoint column. Status
+// commits and checkpoint persistence are split: PGCheckpointer.Save is
+// the only writer of checkpoint and ClearCheckpoint is the only path
+// to remove it. This prevents a status update from accidentally erasing
+// an in-flight checkpoint saved between Load and Update.
 func (e *AgentRun) Update(
 	ctx context.Context,
 	tx pg.Tx,