fix(gc): add nil check when processing post commands

Skip nil entries in the post commands list to prevent potential nil
pointer dereference when accessing RecordingTime.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: AnnatarHe

commands/gc.go

@@ -94,6 +94,9 @@ func commandGC(c *cli.Context) error {
 
 	// save all the data that before cursor
 	for _, cmd := range postCommands {
+		if cmd == nil {
+			continue
+		}
 		if cmd.RecordingTime.After(lastCursor) {
 			newPostCommandList = append(newPostCommandList, cmd)
 		}