[Feature Request] Support switching between multiple dbt platform projects from a single local checkout (Fusion CLI)

### Context

Some teams deploy the same dbt models from one git repo to multiple separate dbt platform projects (e.g. one project per region or per production target sharing a single codebase).

The legacy dbt CLI (dbt platform CLI, `dbt-labs/dbt-cli` formula) supports this pattern by keeping multiple project configs in `dbt_cloud.yml` and letting the user switch which one is active.

The Fusion CLI (`dbt-labs/dbt` formula) maps one local project directory to exactly one `project-id`, set in `dbt_project.yml`'s `dbt-cloud` block. There is currently no native way to target a different dbt platform project from the same local checkout without maintaining a separate project directory per `project-id`.

### Ask

Native support (CLI flag, subcommand, or config override) for targeting a different dbt platform `project-id` per invocation, without requiring N separate local project directories for N platform projects.

### Current workarounds

- Maintaining a separate local directory per `project-id`
- Community tool `dbt-switch`, which swaps the active project in `dbt_cloud.yml`
- Custom wrapper scripts that dispatch based on a marker file per directory

### Reported via

dbt Labs Support (customer-reported feedback)