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

README.md

@@ -90,6 +90,24 @@ source $KEEP_BASH_PROFILE
 
 The modulefile prepends `keep/bin` to `PATH` and sets `KEEP_BASH_PROFILE` pointing to `profile.bash`.
 
+### Shell Completion
+
+Tab completion is available for `bash`, `zsh`, `fish`, `elvish`, and `powershell`.
+
+**Bash** — add to `~/.bashrc`:
+
+```sh
+. <(keep --generate-completion bash)
+```
+
+**Zsh** — add to `~/.zshrc`:
+
+```sh
+. <(keep --generate-completion zsh)
+```
+
+**With `profile.bash`**: Completions for `keep`, `@` (save), and `@@` (get) are loaded automatically when sourcing `profile.bash`.
+
 ### Build with Server/Client Features
 
 ```sh