Add sender sync workflow for shared OpenAPI artifacts

Implement the sender side of the shared OpenAPI artifact sync so TinyPen 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 TinyPen
- Treat the TinyPen 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/tinypen-shared-components.openapi.yaml

Receiver target:
- schemas/openapi/tinypen-shared-components.openapi.yaml

Acceptance criteria:
- TinyPen 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