mciasctl: convert from flag to cobra

Adds shell completion support (zsh, bash, fish) via cobra's built-in
completion command. All existing behavior and security measures are
preserved.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/spf13/pflag/.golangci.yaml

@@ -0,0 +1,4 @@
+linters:
+    disable-all: true
+    enable:
+        - nolintlint