feat: Implement error handling framework (Task 4)

- Add AppError with error codes and hints system
- Implement ConnectionError for SSH-specific errors
- Add HandleSSHError for intelligent SSH error parsing
- Add FormatConnectionError for user-friendly output
- All 5 test functions passing (TestAppError, TestConnectionError,
  TestHandleSSHError, TestHandleSSHErrorNil, TestFormatConnectionError)
- Update PROJECT_STATE.md to reflect Task 4 completion

Progress: Tasks 1-4 complete (~30%)

test/errors_test.go

@@ -0,0 +1,142 @@
+package errors_test
+
+import (
+	stderrors "errors"
+	"strings"
+	"testing"
+
+	apperrors "git.tukangketik.id/swanadiva/hostkeeper/internal/errors"
+)
+
+func TestAppError(t *testing.T) {
+	cause := stderrors.New("underlying issue")
+	appErr := apperrors.NewAppError(
+		apperrors.ErrAuthFailed,
+		"Authentication failed",
+		cause,
+		[]string{"Check credentials", "Verify key permissions"},
+	)
+
+	if appErr.Code != apperrors.ErrAuthFailed {
+		t.Errorf("Expected code '%s', got '%s'", apperrors.ErrAuthFailed, appErr.Code)
+	}
+
+	if len(appErr.Hints) != 2 {
+		t.Errorf("Expected 2 hints, got %d", len(appErr.Hints))
+	}
+
+	// Test error message is non-empty
+	if appErr.Error() == "" {
+		t.Error("Expected non-empty error message")
+	}
+
+	// Test Unwrap
+	if !stderrors.Is(appErr, cause) {
+		t.Error("Expected errors.Is to match the cause via Unwrap")
+	}
+}
+
+func TestConnectionError(t *testing.T) {
+	connErr := apperrors.NewConnectionError(
+		"auth",
+		"Authentication failed",
+		"ssh: handshake failed",
+		[]string{"Check credentials", "Verify key permissions"},
+	)
+
+	if connErr.Type != "auth" {
+		t.Errorf("Expected type 'auth', got '%s'", connErr.Type)
+	}
+
+	if len(connErr.Hints) != 2 {
+		t.Errorf("Expected 2 hints, got %d", len(connErr.Hints))
+	}
+
+	if connErr.Error() == "" {
+		t.Error("Expected non-empty error message")
+	}
+}
+
+func TestHandleSSHError(t *testing.T) {
+	tests := []struct {
+		name         string
+		inputErr     error
+		expectedType string
+	}{
+		{
+			name:         "connection refused",
+			inputErr:     stderrors.New("dial tcp: connection refused"),
+			expectedType: "network",
+		},
+		{
+			name:         "authentication failed",
+			inputErr:     stderrors.New("ssh: handshake failed: ssh: unable to authenticate"),
+			expectedType: "auth",
+		},
+		{
+			name:         "timeout",
+			inputErr:     stderrors.New("dial tcp: connection timed out"),
+			expectedType: "timeout",
+		},
+		{
+			name:         "no such host",
+			inputErr:     stderrors.New("dial tcp: lookup: no such host"),
+			expectedType: "config",
+		},
+		{
+			name:         "permission denied",
+			inputErr:     stderrors.New("ssh: permission denied"),
+			expectedType: "auth",
+		},
+		{
+			name:         "unknown error",
+			inputErr:     stderrors.New("something went wrong"),
+			expectedType: "unknown",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			connErr := apperrors.HandleSSHError(tt.inputErr)
+			if connErr == nil {
+				t.Fatal("Expected non-nil ConnectionError")
+			}
+			if connErr.Type != tt.expectedType {
+				t.Errorf("Expected type '%s', got '%s'", tt.expectedType, connErr.Type)
+			}
+			if len(connErr.Hints) == 0 {
+				t.Error("Expected at least one hint")
+			}
+		})
+	}
+}
+
+func TestHandleSSHErrorNil(t *testing.T) {
+	result := apperrors.HandleSSHError(nil)
+	if result != nil {
+		t.Error("Expected nil for nil input")
+	}
+}
+
+func TestFormatConnectionError(t *testing.T) {
+	connErr := apperrors.NewConnectionError(
+		"auth",
+		"Authentication failed",
+		"ssh: handshake failed",
+		[]string{"Check credentials", "Verify key permissions"},
+	)
+
+	output := apperrors.FormatConnectionError(connErr)
+
+	if output == "" {
+		t.Error("Expected non-empty formatted output")
+	}
+
+	if !strings.Contains(output, "Authentication failed") {
+		t.Error("Expected output to contain error message")
+	}
+
+	if !strings.Contains(output, "Possible solutions") {
+		t.Error("Expected output to contain hints section")
+	}
+}