docs: cross-rung contract pointers in README (F5 SLICE-5, #14)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -98,6 +98,39 @@ CI.
 Design: `vscode-cowriting-plugin-content/specs/coauthoring-propose-accept.md`.
 Live smoke: [`docs/MANUAL-SMOKE-F4.md`](docs/MANUAL-SMOKE-F4.md).
 
+## F5 — Cross-rung format + round-trip (Feature #14)
+
+The `.threads/` sidecar is the **published cross-rung contract**: any rung of
+the ladder (this editor → Gitea substrate → rfc-app) reads and writes the same
+record per the contract; git push/pull is the transport, no re-homing ever.
+
+- **Normative contract (INV-14):**
+  `vscode-cowriting-plugin-content/specs/coauthoring-sidecar-contract.md` —
+  format changes land there first, then schema, then code.
+- **Machine-checkable half:**
+  [`schemas/coauthoring-sidecar.schema.json`](schemas/coauthoring-sidecar.schema.json);
+  validate any sidecar from any rung with
+  `node scripts/validate-sidecar.mjs <file>`. The unit suite validates every
+  serialized artifact — contract drift fails CI.
+- **Identity crosses rungs:** `Provenance.email` (git's own join key —
+  populated from the workspace git config when available) and
+  `agent.onBehalfOf` (who the machine acted for).
+- **Writers preserve unknown fields (INV-15):** rewriting a sidecar never
+  destroys another rung's data — unknown keys survive, after known keys,
+  sorted.
+- **Newer-major sidecars are read-only (INV-16):** the editor renders what it
+  understands, warns once, and writes nothing (the store refuses as a
+  backstop).
+- **Deterministic merge (INV-17):** `src/mergeArtifacts.ts` — union-by-id,
+  documented tie-breaks, every resolved divergence surfaced in `conflicts`.
+- **The round-trip is proven, not asserted:**
+  [`scripts/crossrung-reply.mjs`](scripts/crossrung-reply.mjs) is a
+  self-contained conforming foreign writer (the Gitea-rung stand-in); host E2E
+  drives editor-thread → stand-in reply → external-change → the reply renders
+  in the thread.
+
+Design: `vscode-cowriting-plugin-content/specs/coauthoring-cross-rung-format.md`.
+
 ## Develop
 
 - `npm run watch` — rebuild on change.