feat(cli): shell completion scripts for bash/zsh/fish/powershell (#68)

Implements issue #26. Adds 'distill completion [shell]' using Cobra's
built-in completion generators. Includes per-shell installation
instructions in the command's long description.

Co-authored-by: Ona <no-reply@ona.com>

Author: Siddhant-K-code

cmd/completion.go

@@ -0,0 +1,58 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var completionCmd = &cobra.Command{
+	Use:   "completion [bash|zsh|fish|powershell]",
+	Short: "Generate shell completion scripts",
+	Long: `Generate shell completion scripts for the Distill CLI.
+
+Bash:
+  # One-time setup (requires bash-completion package):
+  $ distill completion bash > /etc/bash_completion.d/distill
+
+  # Per-session:
+  $ source <(distill completion bash)
+
+Zsh:
+  # If shell completion is not already enabled, run:
+  $ echo "autoload -U compinit; compinit" >> ~/.zshrc
+
+  $ distill completion zsh > "${fpath[1]}/_distill"
+
+Fish:
+  $ distill completion fish > ~/.config/fish/completions/distill.fish
+
+PowerShell:
+  PS> distill completion powershell | Out-String | Invoke-Expression
+
+  # To load completions for every new session, run:
+  PS> distill completion powershell > distill.ps1
+  # and source this file from your PowerShell profile.
+`,
+	ValidArgs: []string{"bash", "zsh", "fish", "powershell"},
+	Args:      cobra.ExactArgs(1),
+	RunE: func(cmd *cobra.Command, args []string) error {
+		switch args[0] {
+		case "bash":
+			return rootCmd.GenBashCompletion(os.Stdout)
+		case "zsh":
+			return rootCmd.GenZshCompletion(os.Stdout)
+		case "fish":
+			return rootCmd.GenFishCompletion(os.Stdout, true)
+		case "powershell":
+			return rootCmd.GenPowerShellCompletionWithDesc(os.Stdout)
+		default:
+			return fmt.Errorf("unsupported shell %q: choose bash, zsh, fish, or powershell", args[0])
+		}
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(completionCmd)
+}