Give the shell an echo, so a script can say what it is doing

Say.sbx has printed words since long before there were scripts, and is the
wrong shape for one. It is a program: it has to be found on the disk,
loaded and started, it prefixes what it was told with "it says:", and the
system prints "finished" after it. Three lines of noise around one line of
narration, and a load off the disk to produce them.

echo is a command, so it costs a comparison. With nothing after it, a blank
line - which is what anybody expects and is worth having to space a long
script out.

Not called "say", although that is the better word. Built-in commands are
tried before the disk and always win, so a built-in say would shadow Say.sbx
and quietly change what every existing script and test meant by it.

Also puts "do" in the help, which the commit that added it forgot, and
splits the help text: the new lines pushed it over the assembler's 255
character limit for one string. That failure was hidden for a few minutes
by a 'make' whose errors were going to /dev/null - the build kept the disk
it already had, and the machine cheerfully reported "I do not know: echo"
from a system assembled before echo existed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Tests/expected/cosmosScript.out

@@ -1,7 +1,11 @@
 CosmOS
-> > Say from a script
+> > echo saying what it is doing
+saying what it is doing
+> Say from a script
 it says: from a script
 finished
+> echo
+
 > Say and again
 it says: and again
 finished