docs(f6): manual smoke + README pointer (SLICE-4)

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

docs/MANUAL-SMOKE-F6.md

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+# Manual smoke — F6 diff-view toggle (live)
+
+Pre-req: a real machine (not CI). Step 4 uses a live Claude turn — Claude Code
+installed + signed in (the `claude-code` provider rides that login, INV-8). The
+rest of F6 needs no credentials and no network.
+
+1. `npm run build`, launch the extension (F5 in VS Code opens the committed
+   `sandbox/` playground), open `playground.md`.
+2. Edit a sentence by hand, then **Cmd/Ctrl-Alt-D** (or run **Cowriting: Toggle
+   Diff View**). ✅ A diff opens: the readonly baseline on the left, your live
+   document on the right; the tab title reads `playground.md — my changes since
+   opened <time>`. Your hand edit shows as a change.
+3. Keep typing **in the diff's right pane**. ✅ Editing continues in place;
+   decorations/threads still work. Toggle again. ✅ The diff tab closes and the
+   normal editor is back.
+4. Select a sentence → right-click → **Ask Claude to Edit Selection** → give an
+   instruction → **✓ Accept Proposal**. Now toggle the diff. ✅ The accepted
+   text is **not** shown as a change (the baseline advanced past the landing —
+   INV-18); only your own subsequent edits show.
+5. Run **Cowriting: Pin Diff Baseline to Now**. ✅ The diff empties (baseline =
+   now); the title reads `pinned <time>`. Type → your changes show against the
+   pin.
+6. Reload the window (Developer: Reload Window). ✅ Toggle: the baseline is the
+   same as before the reload (persisted in workspace storage, not the repo —
+   INV-19; `git status` shows nothing new).
+7. Toggle on a file outside the workspace folder. ✅ A warning, no diff.