test: add comprehensive test suite for engine, agent and cmd packages

- Refactor download.go and stream.go with downloadDeps/streamDeps structs
  for dependency injection, enabling unit testing without real I/O
- download_test.go: 15 tests — input validation, mock downloaders, method
  selection, cobra Args, deadlock detection
- stream_test.go: input validation, noOpen flag, engine error handling
- client_test.go: context cancellation, timeout, full Sync roundtrip,
  watch-progress and HTTP error unwrapping
- sync_test.go: TriggerSync on watching transition, adjustInterval
- torrent_test.go: TorrentDownloader lifecycle without network
- stream_server_test.go: HTTP server lifecycle, SetFile/ClearFile,
  concurrent requests, Shutdown releases port, content-type
- manager_integration_test.go: full pipeline — success, torrent→debrid
  fallback, all-fail, multi-concurrent, ForceStart, OnTaskDone,
  recent-finished drain, cancel mid-download, organize
- usenet_test.go: Cancel/Pause race regression test (run with -race)
- daemon_test.go: isAllowedStreamPath table tests
- CI: split coverage gate to engine+agent only (50% threshold); cmd
  coverage still reported but not gated (interactive UI commands)
- lefthook: add pre-push hook with go test -race -count=1 -timeout=120s

.github/workflows/ci.yml

@@ -75,8 +75,32 @@ jobs:
         with:
           go-version: "1.25"
 
-      - name: Run tests with coverage
-        run: go test -race -coverprofile=coverage.out -covermode=atomic ./...
+      - name: Run tests with coverage (all packages)
+        run: |
+          go test -race -coverprofile=coverage.out -covermode=atomic \
+            ./internal/engine/... \
+            ./internal/agent/... \
+            ./internal/cmd/...
+
+      - name: Check coverage threshold (engine + agent)
+        run: |
+          # Threshold applies only to engine and agent — cmd contains interactive UI
+          # commands (config menus, daemon, auth browser) that are not unit-testable.
+          go test -race -coverprofile=coverage-core.out -covermode=atomic \
+            ./internal/engine/... \
+            ./internal/agent/...
+          COVERAGE=$(go tool cover -func=coverage-core.out | grep ^total | awk '{print $3}' | tr -d '%')
+          echo "Coverage on engine+agent: ${COVERAGE}%"
+          python3 -c "
+          coverage = float('${COVERAGE}')
+          threshold = 50.0
+          print(f'Coverage: {coverage:.1f}% (threshold: {threshold}%)')
+          if coverage < threshold:
+              print(f'ERROR: Coverage {coverage:.1f}% is below minimum {threshold}%')
+              exit(1)
+          else:
+              print('OK: Coverage meets minimum threshold')
+          "
 
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v6