feat(terminal): restrict default allowed commands to 'ls' and 'echo'

[RonithManikonda]

Change the default value of terminal.allowCommands from an empty array to ['ls', 'echo']. This change prevents users from running all available commands by default in tutorials, enhancing security and focus. Lesson authors can still allow all commands by specifying terminal.allowCommands: [] in the metadata.

BREAKING CHANGE: The default value of terminal.allowCommands is now restricted to ['ls', 'echo']. To allow all commands, explicitly set terminal.allowCommands: [] in the metadata.

Closes #302