fix: add top-level show command alias

[cyq1017]

Summary:

• Add top-level show as a thin alias for existing config show.
• Reuse the existing config-show handler, args validation, completion, and --json behavior.
• Add focused tests for command registration and root help visibility.

Fixes #332

Verification:

• /Volumes/T7_500G/agent-open-source-contrib/toolchains/go1.24.6/bin/go test ./cmd/container-use -run 'Show|Config' -count=1
• git diff --check

Note: broad go test ./... -short -count=1 was attempted earlier but stalled in this environment, so validation is focused on the touched command path.