feat(cli): add shell completion support (PSGO-200)

Enable Cobra's built-in shell completion for bash, zsh, fish, and
PowerShell. The command is hidden from `kbc --help` (HiddenDefaultCmd)
but accessible via `kbc completion <shell>`.

Skip the full dependency stack initialization (filesystem, env loading,
HTTP version check) for completion commands, as every TAB press invokes
`__complete` and the heavy init causes side effects and latency.

Closes #2544
Linear: PSGO-200

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Matovidlo

internal/pkg/service/cli/cmd/cmd.go

@@ -118,7 +118,7 @@ func NewRootCommand(stdin io.Reader, stdout io.Writer, stderr io.Writer, osEnvs
 		Use:               "kbc", // name of the binary
 		Version:           version.Version(),
 		Short:             helpmsg.Read(`app`),
-		CompletionOptions: cobra.CompletionOptions{DisableDefaultCmd: true},
+		CompletionOptions: cobra.CompletionOptions{HiddenDefaultCmd: true},
 		SilenceUsage:      true,
 		SilenceErrors:     true, // custom error handling, see printError
 		RunE: func(cmd *cobra.Command, args []string) (cmdErr error) {
@@ -145,6 +145,12 @@ func NewRootCommand(stdin io.Reader, stdout io.Writer, stderr io.Writer, osEnvs
 	// Init when flags are parsed
 	p := &dependencies.ProviderRef{}
 	root.PersistentPreRunE = func(cmd *cobra.Command, args []string) error {
+		// Completion commands don't need the full dependency stack.
+		// __complete is the hidden command invoked by the shell on every TAB press.
+		if cmd.Name() == "completion" || cmd.Name() == "__complete" {
+			return nil
+		}
+
 		// Bind flags - without ENVs from files
 		root.globalFlags = flag.DefaultGlobalFlags()
 		err := cmdconfig.NewBinder(osEnvs, log.NewNopLogger()).Bind(cmd.Context(), cmd.Flags(), args, &root.globalFlags)