F3: document live attribution + test commands (#6)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -11,6 +11,9 @@ Registers one command, **`Cowriting: Show Cline SDK Info`**, which loads
 catalog (a pure, key-free SDK call) in a notification and the
 "Cowriting (Cline SDK)" output channel.
 
+Features shipped so far: F2 region-anchored threads (Feature #4), F3 live
+human/Claude attribution (Feature #6).
+
 ## Architecture
 
 - CommonJS extension bundled with esbuild (`src/extension.ts` → `out/extension.cjs`).
@@ -43,10 +46,40 @@ sidecars under `.threads/<doc-path>.json` (plain, diffable JSON — no server).
 Design: `vscode-cowriting-plugin-content/specs/coauthoring-inner-loop.md`. No live
 `@cline/sdk` turn and no credentials are involved in F2.
 
+## F3 — Live human/Claude attribution (Feature #6)
+
+As you and Claude coauthor, every span in the buffer carries an author: human
+edits render with a subtle left border, Claude-authored spans with a background
+tint. Text that predates tracking stays plain — the honest record. Edits that
+touch a boundary (split, merge, partial overwrite) are handled char-precisely.
+
+Attribution is persisted git-natively in the same `.threads/` sidecars
+(`attributions[]`, sharing the same `anchors` fingerprints as F2 threads). On
+reload, fingerprints re-resolve spans against the current document; spans that
+can't be confidently re-found are **orphaned** (status-bar count + "Cowriting
+Attribution" output channel) rather than silently moved or discarded.
+
+**Commands**
+
+- **`Cowriting: Ask Claude to Edit Selection`** — select text → enter an
+  instruction → a live `@cline/sdk` turn runs on the built-in `claude-code`
+  provider (rides your local Claude Code Pro/Max login; the extension stores no
+  credentials). The replacement lands in the buffer as a Claude-attributed span.
+- **`Cowriting: Toggle Attribution`** — show/hide attribution decorations.
+- `cowriting.applyAgentEdit` _(palette-hidden)_ — the single machine-edit
+  ingress seam. Tests drive this directly so CI requires no LLM.
+
+Design: `vscode-cowriting-plugin-content/specs/coauthoring-attribution.md`.
+
 ## Develop
 
 - `npm run watch` — rebuild on change.
-- `npm test` — vitest unit suite (SDK driver, schema, store, anchorer, thread mutations).
+- `npx vitest run` — unit suite (SDK driver, schema, store, anchorer, thread
+  mutations, attribution split/merge).
 - `npm run test:e2e` — `@vscode/test-electron` host E2E
-  (create → reply → resolve → persist → reload → re-anchor → orphan).
+  (create → reply → resolve → persist → reload → re-anchor → orphan; drives
+  `cowriting.applyAgentEdit` directly — no LLM required).
+- `npm run smoke:live` — scripted live-turn smoke test for F3; requires Claude
+  Code installed and signed in. See
+  [`docs/MANUAL-SMOKE-F3.md`](docs/MANUAL-SMOKE-F3.md).
 - `npm run typecheck` — type-check without emit.