Add sender sync workflow for shared OpenAPI artifacts

[cubap]

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