Add TinyNode-owned shared OpenAPI artifact sync workflow

[Copilot]

This introduces the sender side of shared OpenAPI artifact sync so TinyNode owns the canonical shared contract in-repo and can propagate updates into cubap/rerum_openapi via an automated PR. It standardizes the provider artifact path while preserving the existing receiver-side validation surface.

• Canonical shared artifact

  • Adds the exported TinyNode contract at openapi/components/tinynode-shared-components.openapi.yaml
  • Mirrors the current receiver artifact so future edits happen in TinyNode, not downstream

• Cross-repo sync workflow

  • Adds TinyNode Shared OpenAPI Sync workflow
  • Triggers on changes to the canonical artifact on main and via manual dispatch
  • Checks out cubap/rerum_openapi, verifies the receiver stub exists, copies the artifact, and opens/updates a PR against main

• Repository guardrails

  • Extends the existing repo file presence test to cover the new canonical artifact and workflow
  • Documents the canonical source path and required sync token in README.md

Example workflow shape:

on:
  push:
    branches: [main]
    paths:
      - openapi/components/tinynode-shared-components.openapi.yaml

jobs:
  sync:
    steps:
      - uses: actions/checkout@v4
      - uses: actions/checkout@v4
        with:
          repository: cubap/rerum_openapi
          path: receiver
      - run: cp openapi/components/tinynode-shared-components.openapi.yaml \
               receiver/schemas/openapi/tinynode-shared-components.openapi.yaml
      - uses: peter-evans/create-pull-request@v8