feat(userspace): shell runs programs (ls, cat) via spawn w/ terminal+storage+args page; ext4 holes read as zeros; storage re-attach same buffer

disk/docs/shell.txt

@@ -0,0 +1,7 @@
+The shell runs a program by asking init to load it (init alone may
+create processes), then gives the program the terminal as its output
+and the storage server, writes its arguments onto a page it moves
+into the program, starts it and waits for it to die.
+
+Built-ins: help, echo, clear. Anything else is a program on disk:
+`name args...` runs name.elf.