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.zsh

@@ -0,0 +1,38 @@
+#!/bin/zsh
+
+autoload -U add-zsh-hook
+
+__keep_preexec() {
+  KEEP_META_command="$1"
+  KEEP_META_tty=${KEEP_META_tty:-$(tty)}
+}
+
+add-zsh-hook preexec __keep_preexec
+
+keep() {
+  if [[ $ZSH_SUBSHELL -le 2 ]]; then
+    export KEEP_META_command
+  fi
+  export KEEP_META_tty
+  command keep "$@"
+}
+
+alias @='keep --save'
+alias @@='keep --get'
+
+# Shell completions
+. <(command keep --generate-completion zsh)
+
+___keep_complete() {
+    local mode="$1"
+    local -a words
+    words=(keep "$mode" "${words[@]:1}")
+    ((CURRENT++))
+    _keep
+}
+
+___keep_save_completion() { ___keep_complete --save; }
+___keep_get_completion() { ___keep_complete --get; }
+
+compdef ___keep_save_completion @
+compdef ___keep_get_completion @@