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

src/args.rs

@@ -2,6 +2,7 @@ use std::path::PathBuf;
 use std::str::FromStr;
 
 use clap::*;
+use clap_complete::Shell;
 
 /// Main struct for command-line arguments, parsed via Clap.
 #[derive(Parser, Debug, Clone)]
@@ -68,6 +69,10 @@ pub struct ModeArgs {
     #[arg(help("Generate default configuration and output to stdout"))]
     pub generate_config: bool,
 
+    #[arg(help_heading("Mode Options"), long, conflicts_with_all(["save", "get", "diff", "list", "delete", "info", "status", "server", "generate_config"]))]
+    #[arg(help("Generate shell completion script (bash, zsh, fish, elvish, powershell)"))]
+    pub generate_completion: Option<Shell>,
+
     #[arg(help_heading("Server Options"), long, env("KEEP_SERVER_ADDRESS"))]
     #[arg(help("Server address to bind to"))]
     pub server_address: Option<String>,