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/errors.go

@@ -0,0 +1,48 @@
+package errors
+
+import (
+	"fmt"
+)
+
+// Error codes
+const (
+	ErrHostNotFound        = "HOST_NOT_FOUND"
+	ErrAuthFailed          = "AUTH_FAILED"
+	ErrConnectionTimeout   = "CONNECTION_TIMEOUT"
+	ErrInvalidConfig       = "INVALID_CONFIG"
+	ErrKeyNotFound         = "KEY_NOT_FOUND"
+	ErrPermissionDenied    = "PERMISSION_DENIED"
+	ErrFileCorrupted       = "FILE_CORRUPTED"
+	ErrInvalidCredentials  = "INVALID_CREDENTIALS"
+)
+
+// AppError represents an application error with code, message, cause, and hints
+type AppError struct {
+	Code    string
+	Message string
+	Cause   error
+	Hints   []string
+}
+
+// Error implements the error interface
+func (e *AppError) Error() string {
+	if e.Cause != nil {
+		return fmt.Sprintf("[%s] %s: %v", e.Code, e.Message, e.Cause)
+	}
+	return fmt.Sprintf("[%s] %s", e.Code, e.Message)
+}
+
+// Unwrap returns the underlying cause for use with errors.Is/errors.As
+func (e *AppError) Unwrap() error {
+	return e.Cause
+}
+
+// NewAppError creates a new application error
+func NewAppError(code, message string, cause error, hints []string) *AppError {
+	return &AppError{
+		Code:    code,
+		Message: message,
+		Cause:   cause,
+		Hints:   hints,
+	}
+}