Add provider-driven sync for shared RERUM OpenAPI artifacts

[cubap]

Implement a provider-driven sync so RERUM can update a canonical shared OpenAPI artifact in its own repo and have this repo open a sync PR automatically.

Scope:

• Add a workflow in rerum_server_nodejs that triggers when openapi/components/rerum-shared-components.openapi.yaml changes
• Dispatch the sync-provider-artifact workflow in cubap/rerum_openapi
• Sync into schemas/openapi/rerum-shared-components.openapi.yaml
• Keep the target file stubbed here so the workflow updates an existing artifact instead of creating a new one

Acceptance criteria:

• A provider repo change to the exported artifact opens or updates a PR in cubap/rerum_openapi
• Consumers continue validating against this repo without per-feature contract workflows
• The provider path is standard OpenAPI layout and easy to find

Notes:

• The provider repo is public, so the receiver workflow can read the artifact without a separate source token
• If needed, assign this issue to Copilot or the automation owner