Generate and sync Entity action registrars

[thsnkhn]

Problem

The public SDK supports Entity action registrars, but generated projects have no command or sync path for them. An App author must create registration code and edit the project runner by hand. Hook and Job code already has a supported scaffold and runner-sync workflow.

Required behavior

• Add a command to generate an Entity action handler and registrar for an existing Entity.
• Extend runner discovery and sync to include action registrars.
• Keep generated wiring deterministic and safe to run more than once.
• Do not overwrite App-owned handler code.
• Extend doctor and validation output to report missing or stale action wiring.

Acceptance criteria

• One CLI command creates a valid action scaffold for an existing Entity.
• The project runner registers the action without a manual edit.
• Sync repairs generated wiring after an action package is added or removed.
• Dry-run shows planned files and runner changes.
• Existing Hook and Job generation behavior does not change.