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

Cargo.toml

@@ -18,6 +18,7 @@ thiserror = "2.0"
 base64 = "0.22"
 chrono = { version = "0.4", features = ["serde"] }
 clap = { version = "4.6", features = ["derive", "env"] }
+clap_complete = "4"
 config = "0.15"
 ctor = "0.2"
 directories = "6.0"