The shell runs an app by asking the permission manager, passing along
the terminal as its output and a read-only grant to the current folder:
all the shell has to give. The permission manager adds what the app's
manifest asks for and you allowed, and runs it. Ctrl+C stops it.

You decide what each app may use: `install <folder>` asks you, `apps`
shows what each may use, `allow` and `revoke` change it (revoking from
a running app stops it).

Built-ins: help, echo, clear, cd, pwd, apps, install, allow, revoke.