fix(cli):change the placeholder of input during the shell mode

[JayadityaGit]

Summary

When the user enters shell mode, the input placeholder continues to suggest normal chat usage. However, in this mode, the input only accepts shell commands.

As a result:

• Entering regular messages fails.

• Using @ for file path references also fails.

• The UI implies capabilities that are not actually supported in shell mode, which can be confusing for users.

Expected Behavior

Now the placeholder suggests what type of input is valid.

issues

#15136