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%)

internal/errors/connection_errors.go

@@ -0,0 +1,105 @@
+package errors
+
+import (
+	"fmt"
+	"strings"
+)
+
+// ConnectionError represents SSH connection errors with helpful hints
+type ConnectionError struct {
+	Type    string // "auth", "network", "timeout", "config", "unknown"
+	Message string
+	Details string
+	Hints   []string
+}
+
+// Error implements the error interface
+func (e *ConnectionError) Error() string {
+	return fmt.Sprintf("Connection Error (%s): %s\nDetails: %s", e.Type, e.Message, e.Details)
+}
+
+// NewConnectionError creates a new ConnectionError
+func NewConnectionError(errType, message, details string, hints []string) *ConnectionError {
+	return &ConnectionError{
+		Type:    errType,
+		Message: message,
+		Details: details,
+		Hints:   hints,
+	}
+}
+
+// HandleSSHError processes SSH errors and returns user-friendly errors
+func HandleSSHError(err error) *ConnectionError {
+	if err == nil {
+		return nil
+	}
+
+	errStr := err.Error()
+
+	switch {
+	case strings.Contains(errStr, "connection refused"):
+		return &ConnectionError{
+			Type:    "network",
+			Message: "Cannot connect to server",
+			Details: errStr,
+			Hints:   []string{"Check if server is running", "Verify firewall rules", "Confirm hostname and port"},
+		}
+
+	case strings.Contains(errStr, "authentication failed"), strings.Contains(errStr, "unable to authenticate"):
+		return &ConnectionError{
+			Type:    "auth",
+			Message: "Authentication failed",
+			Details: errStr,
+			Hints:   []string{"Verify username and password", "Check SSH key is loaded", "Test with native SSH client"},
+		}
+
+	case strings.Contains(errStr, "timeout"), strings.Contains(errStr, "timed out"):
+		return &ConnectionError{
+			Type:    "timeout",
+			Message: "Connection timeout",
+			Details: errStr,
+			Hints:   []string{"Check network connectivity", "Try increasing timeout", "Verify server is reachable"},
+		}
+
+	case strings.Contains(errStr, "no such host"), strings.Contains(errStr, "hostname"):
+		return &ConnectionError{
+			Type:    "config",
+			Message: "Invalid hostname",
+			Details: errStr,
+			Hints:   []string{"Verify hostname spelling", "Check DNS resolution", "Try IP address instead"},
+		}
+
+	case strings.Contains(errStr, "permission denied"):
+		return &ConnectionError{
+			Type:    "auth",
+			Message: "Permission denied",
+			Details: errStr,
+			Hints:   []string{"Check user permissions on server", "Verify account is not locked", "Check authentication method"},
+		}
+
+	default:
+		return &ConnectionError{
+			Type:    "unknown",
+			Message: "Connection failed",
+			Details: errStr,
+			Hints:   []string{"Check host configuration", "Verify network settings", "Test with standard SSH client"},
+		}
+	}
+}
+
+// FormatConnectionError formats connection error for user-friendly display
+func FormatConnectionError(err *ConnectionError) string {
+	var output strings.Builder
+
+	output.WriteString(fmt.Sprintf("❌ Connection Error: %s\n\n", err.Message))
+	output.WriteString(fmt.Sprintf("Details: %s\n\n", err.Details))
+
+	if len(err.Hints) > 0 {
+		output.WriteString("Possible solutions:\n")
+		for i, hint := range err.Hints {
+			output.WriteString(fmt.Sprintf("  %d. %s\n", i+1, hint))
+		}
+	}
+
+	return output.String()
+}