feat: add shell completion for host names (#37)

- Add ValidArgsFunction to RootCmd for dynamic host completion
- Add 'sshm completion' subcommand for bash/zsh/fish/powershell
- Support prefix matching and case-insensitive filtering
- Respect --config flag for custom SSH config files
- Add comprehensive tests for completion functionality
- Document setup instructions in README

Co-authored-by: Guillaume Archambault <67098259+Gu1llaum-3@users.noreply.github.com>

cmd/completion.go

@@ -0,0 +1,60 @@
+package cmd
+
+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 sshm.
+
+To load completions:
+
+Bash:
+  $ source <(sshm completion bash)
+  
+  # To load completions for each session, add to your ~/.bashrc:
+  # echo 'source <(sshm completion bash)' >> ~/.bashrc
+
+Zsh:
+  $ source <(sshm completion zsh)
+  
+  # To load completions for each session, add to your ~/.zshrc:
+  # echo 'source <(sshm completion zsh)' >> ~/.zshrc
+
+Fish:
+  $ sshm completion fish | source
+  
+  # To load completions for each session:
+  $ sshm completion fish > ~/.config/fish/completions/sshm.fish
+
+PowerShell:
+  PS> sshm completion powershell | Out-String | Invoke-Expression
+  
+  # To load completions for each session, add to your PowerShell profile:
+  # Add-Content $PROFILE 'sshm completion powershell | Out-String | Invoke-Expression'
+`,
+	DisableFlagsInUseLine: true,
+	ValidArgs:             []string{"bash", "zsh", "fish", "powershell"},
+	Args:                  cobra.MatchAll(cobra.ExactArgs(1), cobra.OnlyValidArgs),
+	RunE: func(cmd *cobra.Command, args []string) error {
+		switch args[0] {
+		case "bash":
+			return cmd.Root().GenBashCompletionV2(os.Stdout, true)
+		case "zsh":
+			return cmd.Root().GenZshCompletion(os.Stdout)
+		case "fish":
+			return cmd.Root().GenFishCompletion(os.Stdout, true)
+		case "powershell":
+			return cmd.Root().GenPowerShellCompletionWithDesc(os.Stdout)
+		}
+		return nil
+	},
+}
+
+func init() {
+	RootCmd.AddCommand(completionCmd)
+}