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

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+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.

disk/readme.txt

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+Welcome to k-os.
+
+Every program here runs with only the capabilities it was given:
+this file reached your screen because the shell handed `cat` the
+terminal and the storage server, and nothing else.
+
+Try:  ls        ls docs        cat docs/shell.txt