Fix tests (bd-6ss and sub-issues) (#626)

Test coverage improvements for bd-6ss. Fixing failing test assumption in follow-up commit.

cmd/bd/onboard.go

@@ -123,7 +123,7 @@ The old approach of embedding full instructions in AGENTS.md is deprecated
 because it wasted tokens and got stale when bd upgraded.`,
 	Run: func(cmd *cobra.Command, args []string) {
 		if err := renderOnboardInstructions(cmd.OutOrStdout()); err != nil {
-			fmt.Fprintf(cmd.ErrOrStderr(), "Error: %v\n", err)
+			_, _ = fmt.Fprintf(cmd.ErrOrStderr(), "Error: %v\n", err)
 		}
 	},
 }