[Phase 3] Implement practical cage config, images, and update commands

[Saber5656]

Background

In the legacy implementation, config, images, and update are mostly stubs. After the MVP, users need practical commands for inspecting configuration, managing images, and understanding update behavior.

Scope

• Define and implement a minimal cage config get/set/list/edit behavior.
• Define and implement a minimal cage images list/pull/remove/prune behavior.
• Decide whether cage update should be a no-op with guidance during pre-alpha or an implemented command.
• Require confirmation for destructive operations.
• Align errors and output with the CLI UX specification.

Acceptance Criteria

• Stubbed not yet implemented behavior is replaced with concrete subcommand behavior.
• Image remove and prune operations do not run without confirmation.
• If update is not supported during pre-alpha, the command gives clear guidance.
• Help text matches the documented behavior.

References

• cage-demo/docs/cli-ux/cli-ux-spec.md
• cage-demo/src/cli/config.rs
• cage-demo/src/cli/images.rs
• cage-demo/src/cli/update.rs