Ship a coatcheck dev CLI with Topcoat hot reload

[superposition]

Story

As a Rust developer, I want one command that starts Coatcheck and watches relevant crates so I can work without manual process management.

Deliverable

Add a coatcheck dev command that discovers configuration, runs the Topcoat workbench, watches app and component sources, and reports useful URLs and errors.

Acceptance criteria

• One command starts a local workbench.
• Rust, CSS, story, and component changes trigger the correct rebuild.
• Manager state survives shard updates and reload boundaries where possible.
• Port selection and collisions are handled clearly.
• The CLI has integration tests and a documented config file.