phase: 134_sync_cancel

**Phase 134 final verification pass — all green** (all 3 task files already in `complete/`; no implementation was missing)

- Verified: 5-state `SyncStatus` + `cancelled` terminal (error None, final counts kept), `POST /api/sync/cancel` (202/409/403), cooperative `_check_cancelled` boundaries, shutdown `CancelledError` contract preserved, `#sync-cancel-btn` show/hide + neutral "Sync cancelled" settle, 8-branch poll tree, re-attach — code, unit, integration, and E2E layers all present
- `uv run pytest --cov=app --cov-report=term-missing` → **3150 passed**, `app/` coverage **99%** (gate: >90%)
- `uv run pytest tests/e2e/test_sync_cancel.py -v --no-cov` (isolated) → **3 passed** (47.6 s)
- `uv run ruff check .` → clean; `uv run pyright` → 0 errors, 0 warnings

**Completion criteria:** ① cancel endpoint 202/409/403 ✅ (unit + E2E) ② `cancelled` terminal reported by status, error null, counts kept ✅ (integration `test_cancel_mid_run_lands_in_cancelled_then_resyncs_clean`) ③ unrequested-cancel pin ✅ (`test_unrequested_cancel_is_never_swallowed_and_never_converts`) ④ UI lifecycle + clean re-sync (sentinel row, full 40-file import) ✅ (E2E) ⑤ suite/coverage/E2E gates ✅ ⑥ uncancelled runs byte-identical ✅ (all 3150 pre-existing tests green untouched)

- Docs: README.md reviewed in full against code (pages, auth endpoints, every config default vs `app/config.py`, palette 9+8, fail-loud, cancel section) — consistent, no changes needed. AGENTS.md: fixed one pre-existing inaccuracy (rule 2 "move its file" → "move its directory" — phases are directories); everything else verified accurate
- No live infrastructure touched; no deviations

**Next pending phase:** `135_git_source_branch`

README.md

@@ -306,6 +306,18 @@ whole git-source refresh in one click:
   timeout — a clone + embed can legitimately take minutes.
 - One sync at a time: a second trigger while a run is in flight gets a
   `409`.
+- **Cancel**: while a run is in flight, a **Cancel** button appears
+  next to the sync button; clicking it posts `POST /api/sync/cancel`
+  (admin only) and registers a cooperative cancel (`202`) — the run
+  stops at the next pipeline boundary (a git clone/pull already in
+  flight runs to completion) and settles into a neutral `cancelled`
+  terminal state (`error: null` — intent, not failure: a "Sync
+  cancelled" result line, no error banner or modal, the button
+  retry-ready) that the status endpoint reports; with no run in
+  flight the request gets a `409` (the UI treats it as a harmless
+  race — the poll settles). A sync started after a cancel runs clean
+  (the importer's content-hash delta re-imports what the cancel
+  skipped).
 
 ---