Add machine-readable version output (turborg --version --json)

[stefanthearchitect]

Context

turborg --version prints a human string (internal/version/). Scripts, package managers, and the self-update path benefit from a stable machine-readable form.

The task

Add a JSON version output (e.g. turborg --version --json or a version --json subcommand) emitting {"version": "...", "command": "turborg", ...}. Keep the existing human output as the default.

Acceptance

• JSON output is valid and stable-shaped.
• Human output unchanged.
• A test. Coverage ≥90%.

Small, well-scoped starter task.