docs: bring the node API wire contract under version control

docs/contracts/README.md

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+# Cross-service contracts
+
+Interfaces between two or more services. When services communicate, the contract lives here rather
+than in either repository, so neither owns it and both can diff it.
+
+## `nodes_api_v1.yml`
+
+The node ↔ server wire contract, currently **1.1.1**. Three independent implementations depend on
+it: the server in `Tower-Finder`, the node client in `retina-node`, and the conformance harness.
+
+**It is versioned, not frozen.** The changelog at the top of the file is authoritative. A change is
+a coordinated event rather than a silent edit: bump the version, record the change in the changelog,
+and tell whoever is mid-build. If you find a genuine error in it, raise it rather than working
+around it locally.
+
+Superseded versions are kept in `contract-history/` so a change can be diffed.
+
+### Why it lives here now
+
+It previously sat at the root of the `owl` meta-workspace, which is not a git repository. The file
+therefore had no history: no way to review a change, and no commit to cite when one landed. The
+version number in the changelog was a convention held by the people working on it rather than
+anything enforced. Tracked as [86cb2d059](https://app.clickup.com/t/86cb2d059).
+
+Moving it here gives it review, history and a citable commit. The copy under `~/owl` is now a
+working copy; **this is the one to edit**.