feat: add --generate-completion for shell tab completion

- Add clap_complete dependency for bash/zsh/fish/elvish/powershell
- Add --generate-completion <shell> flag that prints completion script to stdout
- profile.bash sources completions via command keep --generate-completion bash
- @ and @@ aliases get completions via wrapper functions that delegate to _keep
- README updated with Shell Completion section

profile.bash

@@ -40,4 +40,22 @@ function @@ {
   keep --get "$@"
 }
 
+# Shell completions
+. <(command keep --generate-completion bash)
+
+___keep_save_completion() {
+    COMP_WORDS=(keep --save "${COMP_WORDS[@]:1}")
+    COMP_CWORD=$((COMP_CWORD + 1))
+    _keep
+}
+
+___keep_get_completion() {
+    COMP_WORDS=(keep --get "${COMP_WORDS[@]:1}")
+    COMP_CWORD=$((COMP_CWORD + 1))
+    _keep
+}
+
+complete -F ___keep_save_completion @
+complete -F ___keep_get_completion @@
+
 __keep_preexec_init