fix(lint): check all error returns in test and production code

Fixed all unchecked error return values to pass errcheck linter:

1. internal/updater/updater_test.go:
   - Added error checks for json.NewEncoder().Encode() calls
   - Added error checks for json.NewDecoder().Decode() calls
   - Added error checks for http.Response usage before checking errors
   - Added error checks for w.Write() in test HTTP handlers

2. internal/updater/updater.go:
   - Added error handling for os.Rename() when restoring backup
   - Added error handling for os.Remove() when cleaning up backup
   - Both use warning messages for non-fatal failures

3. internal/installer/installer.go:
   - Added error handling for i.saveState() with warning message
   - Ensures state save failures don't block error reporting

4. internal/installer/installer_test.go:
   - Added error checks for os.Chdir() in both defer and main call
   - Uses proper error handling in defer function

All changes maintain existing behavior while satisfying linter requirements.
Tests pass: go test -v -race ./... ✅
Vet passes: go vet ./... ✅

🤖 Generated with Claude Code
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: BadRat-in

internal/installer/installer.go

@@ -102,7 +102,9 @@ func (i *Installer) RunStage(stageConfigPath string) error {
 		// Save state even on failure for resume capability
 		state.LastStage = stageConfigPath
 		state.LastUpdate = time.Now()
-		i.saveState(state)
+		if saveErr := i.saveState(state); saveErr != nil {
+			i.ui.Warning("Failed to save state: %v", saveErr)
+		}
 
 		return fmt.Errorf("stage execution failed: %w", err)
 	}

internal/installer/installer_test.go

@@ -295,8 +295,14 @@ path = "~/test"
 
 	// Change to temp dir so LoadVersionsLock finds the file
 	origDir, _ := os.Getwd()
-	defer os.Chdir(origDir)
-	os.Chdir(tmpDir)
+	defer func() {
+		if err := os.Chdir(origDir); err != nil {
+			t.Errorf("Failed to restore directory: %v", err)
+		}
+	}()
+	if err := os.Chdir(tmpDir); err != nil {
+		t.Fatalf("Failed to change to temp directory: %v", err)
+	}
 
 	ui := &mockUI{}
 	installer := NewInstaller(ui, false)

internal/updater/updater.go

@@ -176,12 +176,18 @@ func (u *Updater) Update(release *ReleaseInfo) error {
 	tempFile.Close()
 	if err := os.Rename(tempFile.Name(), currentExe); err != nil {
 		// Restore backup on failure
-		os.Rename(backupPath, currentExe)
+		if restoreErr := os.Rename(backupPath, currentExe); restoreErr != nil {
+			// Log but don't fail - original error is more important
+			fmt.Fprintf(os.Stderr, "Warning: failed to restore backup: %v\n", restoreErr)
+		}
 		return fmt.Errorf("failed to replace binary: %w", err)
 	}
 
 	// Remove backup on success
-	os.Remove(backupPath)
+	if err := os.Remove(backupPath); err != nil {
+		// Non-fatal: backup removal failure doesn't break update
+		fmt.Fprintf(os.Stderr, "Warning: failed to remove backup: %v\n", err)
+	}
 
 	return nil
 }

internal/updater/updater_test.go

@@ -64,7 +64,9 @@ func TestCheckForUpdate_NewerVersionAvailable(t *testing.T) {
 		}
 
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(release)
+		if err := json.NewEncoder(w).Encode(release); err != nil {
+			t.Errorf("Failed to encode response: %v", err)
+		}
 	}))
 	defer server.Close()
 
@@ -109,7 +111,9 @@ func TestCheckForUpdate_AlreadyLatest(t *testing.T) {
 		}
 
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(release)
+		if err := json.NewEncoder(w).Encode(release); err != nil {
+			t.Errorf("Failed to encode response: %v", err)
+		}
 	}))
 	defer server.Close()
 
@@ -131,7 +135,9 @@ func TestCheckForUpdate_SkipDraftRelease(t *testing.T) {
 		}
 
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(release)
+		if err := json.NewEncoder(w).Encode(release); err != nil {
+			t.Errorf("Failed to encode response: %v", err)
+		}
 	}))
 	defer server.Close()
 
@@ -140,11 +146,16 @@ func TestCheckForUpdate_SkipDraftRelease(t *testing.T) {
 
 	// Manually test draft detection
 	req, _ := http.NewRequest("GET", server.URL, nil)
-	resp, _ := updater.httpClient.Do(req)
+	resp, err := updater.httpClient.Do(req)
+	if err != nil {
+		t.Fatalf("Request failed: %v", err)
+	}
 	defer resp.Body.Close()
 
 	var release ReleaseInfo
-	json.NewDecoder(resp.Body).Decode(&release)
+	if err := json.NewDecoder(resp.Body).Decode(&release); err != nil {
+		t.Fatalf("Failed to decode response: %v", err)
+	}
 
 	if !release.Draft {
 		t.Error("Expected draft=true")
@@ -160,19 +171,26 @@ func TestCheckForUpdate_SkipPrereleaseRelease(t *testing.T) {
 		}
 
 		w.Header().Set("Content-Type", "application/json")
-		json.NewEncoder(w).Encode(release)
+		if err := json.NewEncoder(w).Encode(release); err != nil {
+			t.Errorf("Failed to encode response: %v", err)
+		}
 	}))
 	defer server.Close()
 
 	updater := NewUpdater("v0.4.0")
 	updater.httpClient = server.Client()
 
 	req, _ := http.NewRequest("GET", server.URL, nil)
-	resp, _ := updater.httpClient.Do(req)
+	resp, err := updater.httpClient.Do(req)
+	if err != nil {
+		t.Fatalf("Request failed: %v", err)
+	}
 	defer resp.Body.Close()
 
 	var release ReleaseInfo
-	json.NewDecoder(resp.Body).Decode(&release)
+	if err := json.NewDecoder(resp.Body).Decode(&release); err != nil {
+		t.Fatalf("Failed to decode response: %v", err)
+	}
 
 	if !release.Prerelease {
 		t.Error("Expected prerelease=true")
@@ -190,7 +208,9 @@ func TestUpdate_SuccessfulUpdate(t *testing.T) {
 
 	// Create mock server for download
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		w.Write([]byte("new version"))
+		if _, err := w.Write([]byte("new version")); err != nil {
+			t.Errorf("Failed to write response: %v", err)
+		}
 	}))
 	defer server.Close()
 
@@ -359,7 +379,9 @@ func TestVerifyChecksum_FileNotFound(t *testing.T) {
 func TestDownloadFile(t *testing.T) {
 	// Create mock server
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		w.Write([]byte("downloaded content"))
+		if _, err := w.Write([]byte("downloaded content")); err != nil {
+			t.Errorf("Failed to write response: %v", err)
+		}
 	}))
 	defer server.Close()