F4: document propose/accept — manual smoke, README, playground loop (#12)

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

docs/MANUAL-SMOKE-F4.md

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+# Manual smoke — F4 propose/accept (live turn)
+
+Pre-req: Claude Code installed + signed in (the `claude-code` provider rides
+that login — the extension holds no credentials, INV-8). Run on a real machine,
+not CI.
+
+1. `npm run build`, then F5 (the EDH opens the committed `sandbox/` playground).
+2. Open `playground.md`, select a sentence → right-click → **Ask Claude to Edit
+   Selection** → give an instruction.
+3. ✅ A **proposal** appears: the selection gets an amber tint and a "Claude
+   proposes" comment thread shows a `diff` of current → proposed. **The document
+   text is unchanged** (INV-10).
+4. Click **✓ Accept Proposal** in the thread title. ✅ The replacement lands and
+   renders Claude-tinted (the F3 attribution substrate); the proposal disappears.
+5. Repeat with another selection and click **✗ Reject Proposal**. ✅ The document
+   is untouched; the proposal disappears.
+6. Propose again, then save and reload the window. ✅ The pending proposal is
+   restored at its re-resolved range.
+7. Failure path: edit the proposed-on text, then try Accept. ✅ Refused with a
+   warning ("target text changed"); undo your edit → Accept works.