feat: add shell profiles for zsh, sh, csh/tcsh

- profile.bash: simplified preexec_init (early return), extracted
  ___keep_complete helper for @/@@ completion wrappers
- profile.zsh: add-zsh-hook preexec, wrapper function, @/@@ aliases,
  completions via compdef
- profile.sh: POSIX-compatible for sh/dash/ksh. Wrapper function,
  @/@@ aliases. No preexec or completions.
- profile.csh: alias-based keep wrapper, @/@@ aliases. No preexec
  or completions.
- modulefile: adds KEEP_SH_PROFILE, KEEP_ZSH_PROFILE, KEEP_CSH_PROFILE
- README: updated Shell Integration table and Shell Completion section

profile.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+# POSIX-compatible profile for sh, dash, ksh93, pdksh, mksh, and other POSIX shells.
+# Preexec hooks are not available in these shells; KEEP_META_command is not set.
+
+KEEP_META_tty=${KEEP_META_tty:-$(tty)}
+
+keep() {
+  export KEEP_META_tty
+  command keep "$@"
+}
+
+alias @='keep --save'
+alias @@='keep --get'