Add sender sync workflow for shared OpenAPI artifacts

Implement the sender side of the shared OpenAPI artifact sync so TPEN-services can own and export its canonical contract artifact from its own repository, then notify cubap/rerum_openapi for a sync PR.

Scope:
- Add a sender workflow in TPEN-services
- Treat the TPEN-services repo as the editing surface for the canonical shared artifact
- Sync into the matching receiver artifact under cubap/rerum_openapi
- Keep the receiver-side target file stubbed there so sync updates an existing artifact rather than creating it on first run

Suggested provider source path:
- openapi/components/tpen-services-shared-components.openapi.yaml

Receiver target:
- schemas/openapi/tpen-services-shared-components.openapi.yaml

Acceptance criteria:
- TPEN-services can change one canonical exported artifact and open or update a PR in cubap/rerum_openapi
- Consumers continue validating against cubap/rerum_openapi without per-feature contract workflows
- The provider source path follows a standard and discoverable OpenAPI layout