Provider contract `$ref` does not resolve on receiver after sync

## Summary

The provider contract at `openapi/contracts/tpen-services-to-tinypen.openapi.yaml` uses relative `$ref`s into the local TinyNode mirror. The refs resolve correctly inside TinyPen's directory layout but **do not resolve on `cubap/rerum_openapi` after sync**, because the receiver's directory layout places the TinyNode mirror at a sibling path that does not exist relative to the synced contract.

## Where it occurs

`openapi/contracts/tpen-services-to-tinypen.openapi.yaml` lines 22-24:

```yaml
/query:
$ref: ../external/tpen-services-to-tinynode.openapi.yaml#/paths/~1query
/update:
$ref: ../external/tpen-services-to-tinynode.openapi.yaml#/paths/~1update
```

## How `../external/` resolves in each repo

| | Contract lives at | `../external/` resolves to | File exists? |
|---|---|---|---|
| TinyPen (local) | `openapi/contracts/` | `openapi/external/tpen-services-to-tinynode.openapi.yaml` | ✓ yes |
| `cubap/rerum_openapi` (receiver, after sync) | `seams/tpen-services-to-tinypen/openapi/` | `seams/tpen-services-to-tinypen/external/tpen-services-to-tinynode.openapi.yaml` | ✗ no |

On the receiver, the TinyNode mirror lives at a sibling path — `seams/tpen-services-to-tinynode/openapi/baseline.openapi.yaml` — not under `seams/tpen-services-to-tinypen/external/`. Any OpenAPI-aware tool that dereferences `$ref`s against the receiver copy gets a file-not-found for `/query` and `/update`.

## What does and doesn't break

- ✓ Local TinyPen `npm test` — passes (resolves under `openapi/external/`)
- ✓ Receiver file storage — fine (YAML text lands; broken refs sit dormant)
- ✓ `__tests__/provider_contract_surface.test.js` — passes (verifies the literal `$ref` string and that the local mirror exists; does not simulate receiver-side resolution)
- ✗ Any `$ref`-resolving tool run against the receiver copy: Redocly bundle/lint, `swagger-cli bundle`, doc renderers, code generators

## Surfaced when

The first cross-repo sync after #47 landed: https://github.com/cubap/rerum_openapi/pull/7. The synced file has the broken refs visible in the diff.

## Remediation — three options, all single-file commits in this repo

1. **Inline the imported operations** in `openapi/contracts/tpen-services-to-tinypen.openapi.yaml` (recommended). Self-contained, survives any transport, matches how the other baselines on the receiver are structured. Costs a few lines of duplication.
2. **Use a receiver-relative path** like `../../tpen-services-to-tinynode/openapi/baseline.openapi.yaml#/paths/~1query`. Breaks local resolution unless we also reorganize TinyPen's `openapi/external/` to match the receiver layout. Trades one breakage for another — not recommended.
3. **Bundle/dereference at sync time** with `redocly bundle` or `swagger-cli bundle` added as a step in `.github/workflows/sync_tinypen_provider_contract.yaml` before the copy. Cleanest semantically, adds a tooling dependency.

## Acceptance criteria

- The receiver copy of `seams/tpen-services-to-tinypen/openapi/baseline.openapi.yaml` resolves cleanly with `npx @redocly/cli bundle ` (no missing-ref errors).
- TinyPen's existing tests continue to pass.
- A receiver-side bundle check could be added later to catch this class of regression in CI.

## Context

Surfaced during the #47 static review. Out-of-scope for that PR by agreement.