[SCUC 2026] Enable custom commands so use can automate own things

[lasomethingsomething]

User story

As a developer, I want to define project-level custom commands so I can automate team-specific workflows through shopware-cli.

Acceptance criteria

Example direction:

• support commands from ~/.shopware-cli/commands/*.js or project-local commands
• expose project context to the command
• allow shopware-cli project foo to execute a custom command named foo

Readiness checklist

• Acceptance criteria are clearly defined.
• Backward compatibility impact addressed.
• Documentation written.
• Tests added or adjusted accordingly.