docs(f8): manual smoke runbook + README hybrid-model/non-shareability note + plan

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

README.md

@@ -196,6 +196,33 @@ the manual smoke, not the sealed-sandbox E2E.
 Design: `vscode-cowriting-plugin-content/specs/coauthoring-rendered-preview.md`.
 Live smoke: [`docs/MANUAL-SMOKE-F7.md`](docs/MANUAL-SMOKE-F7.md).
 
+## F8 — Out-of-workspace authoring (Feature #25)
+
+"Ask Claude to Edit Selection" (and F2 threads / F3 attribution / F4
+propose-accept) now work on **any** document the editor shows — saved in the
+workspace folder, saved outside it, or untitled — matching the already-universal
+F6 diff and F7 preview. Authoring is no longer gated to in-workspace files, and
+the commands are live even with **no folder open**.
+
+Persistence is **hybrid** (one `SidecarStore` abstraction, routed per-document):
+
+- an **in-workspace** file keeps its committable `.threads/<repo-rel>.json`
+  sidecar, **byte-for-byte unchanged** (INV-2) — the only home the F5 cross-rung
+  contract ever sees;
+- an **out-of-workspace** file or **untitled** buffer stores its coauthoring
+  artifact in VS Code **global storage** keyed by `sha256(uri)` (the same home
+  and key F6's baseline uses, INV-19/24); untitled buffers are **in-memory
+  only** (lost on reload/save).
+
+A global-storage artifact is **not a committed file**, so it is **never
+cross-rung-shareable** (INV-25), and renaming/moving the file **orphans** its
+artifact (the key is the URI hash) — both stated as design contract, not
+discovered later. Routing leaves the in-workspace path untouched, so rollback is
+a plain PR revert with zero data migration.
+
+Design: `vscode-cowriting-plugin-content/specs/coauthoring-out-of-workspace.md`.
+Live smoke: [`docs/MANUAL-SMOKE-F8.md`](docs/MANUAL-SMOKE-F8.md).
+
 ## Develop
 
 - `npm run watch` — rebuild on change.