chore(cli): Relocate developer-only commands under a 'dev' subcommand

[duizendstra]

Problem

The cv system-prompt command is a developer-focused utility intended to be run from within the contextvibes/cli source code. However, it is currently exposed as a top-level command.

This led to a confusing user experience when the command was run within a project that uses the CLI (flow-sdk), as it failed with a file not found error because its internal template files were not present. This blurs the line between the user-facing API and internal development tools.

Proposed Solution

To create a clearer separation of concerns and improve the user experience, all commands intended solely for the development of the CLI itself should be moved under a dedicated dev subcommand.

• Move: cv system-prompt -> cv dev system-prompt
• Move: cv dev alias is already correctly placed and serves as a good model.

Acceptance Criteria

1. A new cv dev parent command exists.
2. The system-prompt command is successfully moved and functions as a subcommand of dev (cv dev system-prompt).
3. The top-level cv system-prompt command is removed.
4. The CLI's help text (cv --help and cv dev --help) is updated to reflect the new command structure.