feat(userspace): programs in /bin w/o .elf; bin manifests (needs = display) so shell runs snake; Ctrl+C kills foreground via terminal interrupt notification; ctrl keys; sleep

disk/bin/snake.manifest

@@ -0,0 +1,2 @@
+# What snake needs beyond the terminal and storage.
+needs = display

disk/docs/shell.txt

@@ -1,8 +1,11 @@
-The shell runs a program by asking init to load it (init alone may
+The shell runs /bin/<name> by asking init to load it (init alone may
 create processes), then gives the program the terminal as its output
 and a read-only storage grant to the current directory, writes its
 arguments onto a page it moves into the program, starts it and waits
-for it to die. Then it takes the grant back.
+for it to die. Then it takes the grant back. Ctrl+C kills it.
 
-Built-ins: help, echo, clear, cd, pwd. Anything else is a program on
-disk: `name args...` runs name.elf.
+A program that needs more says so in /bin/<name>.manifest, e.g.
+`needs = display` for snake, which then gets the screen and keyboard
+until it quits. A need the shell does not know, it refuses to run.
+
+Built-ins: help, echo, clear, cd, pwd.

disk/readme.txt

@@ -5,3 +5,4 @@ this file reached your screen because the shell handed `cat` the
 terminal and read-only access to this directory, and nothing else.
 
 Try:  ls        cd docs        cat shell.txt        cat ../readme.txt
+      ls bin    snake (q quits)    sleep 30, then Ctrl+C