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

lefthook.yml

@@ -23,6 +23,12 @@ pre-commit:
           echo "golangci-lint not installed, skipping (install: https://golangci-lint.run/welcome/install/)"
         fi
 
+pre-push:
+  commands:
+    go-test:
+      glob: "*.go"
+      run: go test -race -count=1 -timeout=120s ./...
+
 commit-msg:
   scripts:
     validate.sh: