fix: add golangci-lint config and fix all lint errors

- Add .golangci.yml with v2 config (errcheck, govet, staticcheck, misspell, etc.)
- Fix 32 errcheck issues across config, discord, ntfy, server packages
- Fix misspelling: cancelled → canceled
- Fix staticcheck: use append(slice...) instead of loop
- Fix staticcheck: remove empty error branch
- Use t.Setenv instead of os.Setenv/Unsetenv in tests
- Update CI workflow: add lint job, release tagging, ntfy notifications

internal/server/server_test.go

@@ -13,7 +13,7 @@ func TestServer_HealthEndpoint_StatusCodes(t *testing.T) {
 	mux := http.NewServeMux()
 	mux.HandleFunc("/health", func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "application/json")
-		w.Write([]byte(`{"status":"ok","healthy":true}`))
+		_, _ = w.Write([]byte(`{"status":"ok","healthy":true}`))
 	})
 
 	req := httptest.NewRequest(http.MethodGet, "/health", nil)
@@ -35,7 +35,7 @@ func TestServer_ReadyEndpoint_StatusCodes(t *testing.T) {
 	mux := http.NewServeMux()
 	mux.HandleFunc("/ready", func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "application/json")
-		w.Write([]byte(`{"status":"ready","ready":true}`))
+		_, _ = w.Write([]byte(`{"status":"ready","ready":true}`))
 	})
 
 	req := httptest.NewRequest(http.MethodGet, "/ready", nil)
@@ -56,7 +56,7 @@ func TestServer_Shutdown(t *testing.T) {
 	}
 
 	// Start in background
-	go srv.ListenAndServe()
+	go func() { _ = srv.ListenAndServe() }()
 
 	// Give it a moment to start
 	time.Sleep(10 * time.Millisecond)
@@ -77,7 +77,7 @@ func TestServer_MetricsEndpoint(t *testing.T) {
 	mux := http.NewServeMux()
 	mux.HandleFunc("/metrics", func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusOK)
-		w.Write([]byte("# metrics here"))
+		_, _ = w.Write([]byte("# metrics here"))
 	})
 
 	req := httptest.NewRequest(http.MethodGet, "/metrics", nil)