feat(cli): add shell completion for bash, zsh, and fish (#31)

## Summary

- New `vaultctl completion <shell>` command (bash, zsh, fish)
- Uses Click's `shell_completion` API
- Works without `.vaultctl.yml` config

## Install

```bash
eval "$(vaultctl completion bash)"    # bash
eval "$(vaultctl completion zsh)"     # zsh
vaultctl completion fish > ~/.config/fish/completions/vaultctl.fish  # fish
```

## Test plan
- [ ] `vaultctl completion bash` outputs valid bash completion
- [ ] `vaultctl completion zsh` outputs valid zsh completion
- [ ] `vaultctl completion fish` outputs valid fish completion
- [ ] Tab completion works after eval

Co-authored-by: Fred Thiele <8555720+f3rdy@users.noreply.github.com>

Author: f3rdy

src/vaultctl/cli.py

@@ -61,7 +61,7 @@ def main(ctx: click.Context, config_path: str | None, vault_file: str | None) ->
     cfg_path = Path(config_path) if config_path else find_config()
 
     if cfg_path is None:
-        if ctx.invoked_subcommand in ("init", "self-update"):
+        if ctx.invoked_subcommand in ("init", "self-update", "completion"):
             ctx.obj = VaultContext(VaultConfig())
             return
         click.echo("Error: No .vaultctl.yml found.", err=True)
@@ -780,6 +780,33 @@ def _print_expiry_warning(w: ExpiryWarning) -> None:
     click.echo(f"  {w.key:<40}  {w.expires}  " + click.style(label, fg=color))
 
 
+@main.command()
+@click.argument("shell", type=click.Choice(["bash", "zsh", "fish"]))
+def completion(shell: str) -> None:
+    """Generate shell completion script.
+
+    \b
+    Install (bash):
+      eval "$(vaultctl completion bash)"
+      vaultctl completion bash >> ~/.bashrc
+
+    Install (zsh):
+      eval "$(vaultctl completion zsh)"
+      vaultctl completion zsh >> ~/.zshrc
+
+    Install (fish):
+      vaultctl completion fish > ~/.config/fish/completions/vaultctl.fish
+    """
+    from click.shell_completion import get_completion_class
+
+    comp_cls = get_completion_class(shell)
+    if comp_cls is None:
+        click.echo(f"Error: Unsupported shell: {shell}", err=True)
+        sys.exit(1)
+    comp = comp_cls(main, {}, "vaultctl", "_VAULTCTL_COMPLETE")
+    click.echo(comp.source())
+
+
 @main.command("self-update")
 @pass_ctx
 def self_update_cmd(_vctx: VaultContext) -> None: