[SCUC 2026] Enable custom commands so user can automate own workflows

### 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.