feat: Implement CLI framework with Cobra

- Add root command with Cobra framework
- Implement shell completion support for bash/zsh/fish/powershell
- Add version command
- Add configuration initialization on startup via PersistentPreRunE
- Include verbose (-v/-vv) and debug flags
- Set up proper error handling and exit codes

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/hostkeeper/completion.go

@@ -0,0 +1,67 @@
+package main
+
+import (
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var completionCmd = &cobra.Command{
+	Use:   "completion [bash|zsh|fish|powershell]",
+	Short: "Generate shell completion script",
+	Long: `Generate shell completion script for hostkeeper.
+
+To load completions:
+
+Bash:
+  $ source <(hostkeeper completion bash)
+
+  # To load completions for each session, execute once:
+  # Linux:
+  $ hostkeeper completion bash > /etc/bash_completion.d/hostkeeper
+  # macOS:
+  $ hostkeeper completion bash > /usr/local/etc/bash_completion.d/hostkeeper
+
+Zsh:
+  # If shell completion is not already enabled in your environment,
+  # you will need to enable it.  You can execute the following once:
+  $ echo "autoload -U compinit; compinit" >> ~/.zshrc
+
+  # To load completions for each session, execute once:
+  $ hostkeeper completion zsh > "${fpath[1]}/_hostkeeper"
+
+  # You will need to start a new shell for this setup to take effect.
+
+fish:
+  $ hostkeeper completion fish | source
+
+  # To load completions for each session, execute once:
+  $ hostkeeper completion fish > ~/.config/fish/completions/hostkeeper.fish
+
+PowerShell:
+  PS> hostkeeper completion powershell | Out-String | Invoke-Expression
+
+  # To load completions for every new session, run:
+  PS> hostkeeper completion powershell > hostkeeper.ps1
+  # and source this file from your PowerShell profile.
+`,
+	DisableFlagsInUseLine: true,
+	ValidArgs:             []string{"bash", "zsh", "fish", "powershell"},
+	Args:                  cobra.ExactValidArgs(1),
+	Run: func(cmd *cobra.Command, args []string) {
+		switch args[0] {
+		case "bash":
+			_ = cmd.Root().GenBashCompletion(os.Stdout)
+		case "zsh":
+			_ = cmd.Root().GenZshCompletion(os.Stdout)
+		case "fish":
+			_ = cmd.Root().GenFishCompletion(os.Stdout, true)
+		case "powershell":
+			_ = cmd.Root().GenPowerShellCompletionWithDesc(os.Stdout)
+		}
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(completionCmd)
+}